Thursday, January 21, 2016

If anything, the snow fall could be even bigger in DC

There's nothing more to add from tonight's guidance that wasn't considered in last night's.  The uncertainty remains about the northern edge of the big snows as the SREF suite still insists NYC to Boston will get a major dump while the GFS keeps its southern solution.

The NAM forecast of total snowfall for a 10 to 1 snow ratio from this evening emulates the narrative the earlier SREF depicts of heavy snow fall well into New England.  New York City to Boston would join in on the fun.  Courtesy Pivotal Weather

Meanwhile the GFS denies an epic snow fall for New York City to Boston.  There is no scenario that denies the DC area an epic snowstorm.  Courtesy of Pivotal Weather

The big snow fall rates of 2-4"/hr will happen tomorrow pm in southwest VA, then spread into the DC to Shenandoah Friday night, finally into southeast PA and adjacent NJ Saturday afternoon.  I favor this because it follows the motion of the 700mb WAA pattern, and perhaps the trough of warm air aloft as the upper-level low expands in size.  However there's evidence that the DC area could be in the pivot region allowing more persistent snow well into Saturday night while at the same time the main system shifts more to the east.

Composite reflectivity and precip type from the high resolution NAM valid 03Z Saturday morning.  The heaviest snow rates will be at the latitude of the DC area.

By 15 Z Saturday the heavy band is forecast to shift north into Southern New England to southeast Pennsylvania.  Snowfall remains in the DC area courtesy of the wrapping elevated and saturated warm air advection.

The snow fall rates alone will be sufficient to overwhelm all but the most frequently plowed highways.  Considering the advertised monster snow on a Friday night, I'm anticipating most roads to be clear of traffic allowing whatever plowing planned to be unhindered.  If you are daring to go out, you may be in light company.

I anticipate this snow to be equivalent to what's experienced in a heavy lake Ontario long axis snow band.  I've experienced this and the following is a quick synopsis of what you would experience.  Basically I will assume you have enough ground clearance to handle up to a foot of snow on the highway though any more than that and you may need a hummer.  Expect the <1/4 to 1/16 mile visibility in large dendrites clusters to rimed snowballs to slow you down to the point that snow will quickly clog up your wind shield wipers.  You will need to stop frequently to give them a whack before starting up again.  Clicking on the high beams will give you the feeling of the Millennium Falcon engaging hyperdrive.  No doubt the sensation will be distracting to say the least.

I'm expecting at some point the excessive mess-alpha scale vertical motion field will sufficient vertical instability to generate lightning, momentarily blinding you in a sheet of brilliant orchid purple.  No doubt you'll expect to hear a blast of thunder but with the insulating effect of billions of frozen hydrometeors the sound will be surprisingly muffled.  If there is to be lightning, the snow may wind up falling as soft graupel the size of nickels to quarters.  Such intense convective frozen precipitation has been documented by Picca et al. (2014) in another epic snowband in Long Island. With temperatures approaching the melting point aloft, some of the grapple may appear refrozen, however the convective nature of the heaviest part of the band will prevent pure sleet.  You'd have to be outside the band to see sleet, especially southeast of the district.  One thing that may happen is that the band will be sufficiently electrified to charge the snow flakes and soft graupel to the point that it will stick to everything.  It's quite likely signs will become caked with snow.  Perhaps your car may as well requiring even more frequent wiping with your glove on the highway shoulder, should you find it.  With the bad visibility and snow-caked highway signs, you'd better have a GPS enabled map to make sure you don't miss the exit should you wish to bail.  If you do bail, make sure you've got 4-wheel drive or chains.  There are plenty of hills that will offer up a free uncontrolled ride into something you don't want to hit.

Forecast sounding for Dulles International Airport from the 00 Z 4km NAM (1/22) valid for 09 Z 1/23.  Note layers of conditional instability in the midlevels sufficient for generating lightning perhaps.

One parameter I don't expect will be a serious problem for your Friday night journey will be gobs of blowing snow in the immediate DC area.  The winds will be probably 10-20 mph in tree-covered residential areas, enough for blowing snow to temporarily blind you.  However you should still be able to drive 15 mph or so without needing a spotter ahead of you to let you know where the edge of the highway happens to be.  In serious lake effect bands the winds can be over 40 mph and that's when I've needed a spotter.  By Saturday morning sunrise, the real pressure gradient will kick in and that's when you'll see the mass aerial migration of accumulated snow on trees, rooftops, and clearings.  Serious whiteouts, of the lake effect kind, will commence and your journey will be accompanied by frequent intermissions while you attempt to figure out where the road went.

Ten meter wind forecast from the 00Z 4km NAM valid for 03 Z Saturday morning.  The winds in the DC residential and urban areas will be < 20 mph for the most part.

Ten meter wind forecast from the 00Z 4km NAM valid for 15 Z Saturday morning.  The winds in the DC residential and urban areas will be > 20 mph for the most part.

At this point, you don't want to just drive around a relatively tame, marginal blizzard.  You want to see true gale force winds resulting in blinding amounts of blowing snow.  For that, you'll need to head closer to the coast.  The problem is that most coastal areas will likely experience ice pellets and thus all you'd experience is the sound of high-velocity peppering of BB's against your windshield.  Instead, you may want to head to coastal NJ, preferably the northern half, south of Barnegat Light, to perhaps  Avalon.  This may be the sweet spot where the easterly low-level jet will smack unimpeded into the coast and then some distance inland while intersecting the cold enough temperatures and intense vertical velocity.   This is the area that I'd expect true blizzard conditions.  However, some of that action may get down to the northern Deleware and Chesapeake bays too.  That's good because if you're meandering around the DC area in the snow, you will not make NJ for a loooong time.  

probability of blizzard from the WPC winter storm impact graphics.  Notice that coastal NJ, northern Deleware are really the only places to be hit.  Coastal New England and Long Island may get blizzard but the SREF may be wrong about the northern extent of the snow.
This is probably a good thing anyway because you may wind up being a little too adventurous and decide to drive into the path of boardwalk-breaching waves or perhaps a falling power pole.  The surge will be impressive anywhere along the Deleware to New Jersey coasts.  With the full moon, the morning high tide will send water up to 4 ft above normal water level at that time which will offer the remnants of the 20' waves offshore to nail you.

Waterlevel forecasts (normal tide in solid, forecast water level in dashed and confidence interval shaded) for Barnegat Light, NJ courtesy of SUNY Stonybrook.

Wednesday, January 20, 2016

The oncoming MidAtlantic blizzard still on track and my pick for the most miserable weather

The latest forecasts for the MidAtlantic blizzard are on track for an extremely heavy snowfall event starting in the DC area by afternoon on Friday.   With no melting expected after the snow flakes land on the ground, I'm expecting roads will quickly become snow-covered within an hour of first snow except where roads have been treated.  And I'd expect more roads to be treated than with todays light snow, big impact event.  But look at how quickly the SREF members pile on the snow within a couple hours of onset.  The heaviest rates appear to occur with the strong surge of elevated warm air advection along the front end of this system Friday night.  I don't imagine any road treatment, except for aggressive plowing, will be able to stay ahead of the accumulating snow.  Considering that everyone will be expecting impassible conditions Friday night, I don't expect big traffic jams and numerous trapped people and thus plowing will not be seriously inhibited.

Forecast SREF snowfall accumulation at Washington DC.

At least the winds will not be strong Friday night and visibilities will be restricted mainly by snow to 1/8 mi.  Don't expect them to stay light by Saturday, however.  While snowfall rates will diminish, roads exposed to fields could experience local whiteout conditions.

The 4km NAM does show one area where strong winds may begin earlier - the Shenandoah valley.  Downslope or gap winds could enhance blowing snow and drop visibilities pretty quickly Friday night in some areas.  Would this impact I-81?  It would be interesting to find out.

Forecast 10 m winds from the NAM 4km model.

By Saturday the storm will be at its peak across the DC area and into Philadelphia to the New Jersey Coast.  What I find impressive is the continuing variability in the thickness and precipitation fields from the short range ensemble from late this afternoon.  The precipitation fields are quite varied amongst the SREF members, with the front precip edge ranging from southern to northern Pennsylvania.  Likewise the snowfall accumulation forecasts are equally variable, more so from points along the northern fringe of the expected precipitation.  Just like last year, I'll be thinking of the challenge facing NWS forecasters from State College to New York City when they have to come up with a snowfall forecast.
SREF forecast of 3 hour precipitation and sea level pressure  for Saturday morning from the Wednesday afternoon run.

If I picked a spot where conditions would be most horrid, it may not be with the purely heaviest snowfall rates that may fall from the Shenandoah to northwest Maryland but instead, the northern New Jersey Coast.  This area on Saturday will likely face the highest chance of blizzard conditions while at the same time the NOAA storm surge model depicts water level departures of more than 4 ft and waves over 20 ft high.

Forecast likelihood of blizzard conditions and >2"/hour snowfall rates for Saturday morning.

Storm surge forecast by Saturday evening Jan 24.

Wave height forecasts from the Wavewatch model Saturday night

High tide at Barnegat, NJ Saturday afternoon could feature wave spillover into houses and boardwalks while wet wind driven snow plasters the sides of houses and covers roads with a gluey mess where the surge fails to reach at the same time bringing down power lines.  Doesn't that sound wonderful?

Waterlevel forecast from SUNY Stonybrook for Barnegat, NJ

Friday, April 3, 2015

Moore damage survey of March 25, 2015

The Moore tornado of March 2015 provided a challenge to damage surveyors to derive an accurate rating.  Initially the NWS Norman office gave a preliminary rating of EF1 which led to some heartache by some of the media and yet allowed the survey teams to reassess the tornado strength.  Eventually the members of the four survey teams met to discuss whether or not the tornado should be upgraded.  There were several areas that were under consideration for an upgrade and the teams decided that the damage to several structures was enough to assign the tornado an EF2 rating and others that did not make the cut. This post provides some of the thoughts that went into deciding what to rate the tornado based on these structures.  Before going on, you may wish to visit the NWS Moore tornado page for detail on the tornado path and also this post about the meteorological aspects of the tornado.  I also highly recommend this blog entry by Robin Tanamachi for a well written story of what what it was like to be surveying this tornado.  This blog post is a more detailed perspective from someone who is a meteorologist trying to understand the point of view of an engineer and a forest ecologist when faced with damage to structures and trees.  I try my best but both the engineer and the ecologist have a much more in-depth understanding of the terms I attempt to use and the application to their fields.  You may find some edits to this post once they begin to read it.  :^)

The Moore tornado began at 2335 UTC on March 25 near Southwest 119th St. and May Ave. in southwest Oklahoma City and ending near Northeast 36th Ave and Indian Hills Road.   As the tornado matured it moved east and then took a turn to the southeast.  The track was surveyed by four teams:  The first covered the beginning quarter of the track (Chris Spannagle, Steve Mullens, Alyssa Bates and Matt Taraldsen), The second looking over the wide portion of the track just west of I-35 included Doug Speheger, Tiffany Meyer and Robin Tanamachi, The third covered the area from I-35 and east for three miles involved Dan Dawson, Robert Prentice and I, and then a team of one, Greg Stumpf, surveyed the final few miles of the track.
Figure 1.  A map of the Moore tornado of March 2015 including damage tracks and also a faded image of the rotation tracks where red indicates azimuthal vorticity of .01 /s or greater.   The three boxes represent areas of 

The first area of interest included the only areas of damage determined to be EF2.  These were in the vicinity of the Southgate elementary school area and then a half mile to the southeast.  Both team one and team 2 surveyed these areas but Robert Prentice and I did a more detailed survey of the Southgate elementary school (labeled '2' below).  The area of interest to me was the house with the highest degree of damage found in the entire tornado track and that is labeled '1' in figure 2.
Figure 2.  An intermediate zoomed view of the most intense portion of the tornado with EF scale contours and individual damage points where blue is EF0, green is EF1 and yellow, EF2.

The Southgate school area contained the most intense portion of the tornado with the maximum width and intensity.  Much of the tornado was dominated by a northwesterly inflow jet from the RFD with occasional corner flow vortices.  The strongest of these vortices hit just west of Southgate elementary and then to the southeast.  I’ll focus on the first one that included Southgate and a couple houses to the west (labeled 1 and 2 in figure 3).
Figure 3.  The highest zoomed view of the first area of interest including the EF2 house (1) and the Southgate elementary school (2a-c).  
 The house in the images below lost several walls once the roof was lifted away (figure 4).  According to the teams surveying this house, there were anchor bolts driven every 8’ to a depth of about 2” along the bottom sill plates into the foundation.  Otherwise there were shot nails driven into the foundation.  Not much is known about the roof-to-wall connections, however the house appears in Google’s street view product and some assessment can be made about the shape of the structure (figure 6).  While the house was embedded in shrubs a gable roof can be identified both on the west and south ends.   Since the debris in the picture facing NW appears to have blown eastward, the west gable was likely exposed to the strongest winds.  It is possible the weak links in the house were the gabled west side of the roof and the garage door seen in the second picture (figure 5).  The expected wind speed for collapsed exterior walls was 132 mph but team 2 lowered the wind speed down to the lower bound 113 mph.  Both lie within the EF2 rating.  The Cleveland County Assessors site indicates that the house was built in 1963. 

The next house to the north also lost its roof though it preserved all of its exterior walls.  The surveyors lowered the estimated wind speed to 107 mph, which is EF1, based on their similar appraisal of weaker than typical construction quality.
Not many other DIs exist in close proximity of these two houses, except for other upstream and downstream houses.  The trees in the pictures were located outside the core region of the tornado thus leaving only the telephone poles as potentially representative confirming damage indicators.  All of them were replaced after the tornado, however a piece of one lay against one of the collapsed walls in the first picture (figure 4).  Broken wooden poles also yields an EF2 rating with an expected wind of 118 mph.

Figure 4.  A photograph of the EF2 house with roof gone and some walls collapsed.   
Figure 5.  A photograph of the same house from its west side facing east.  The garage door blown inward was below a gabled roof end.  The house on the left also lost its entire roof but all walls remained standing.
Figure 6.  A Google street view picture of the house from the same view as the first image of the house above.  

 The Southgate elementary school, built in 1963, lost part of its roof on the southwest end of the building.  This image in figure 6 shows some of the debris that landed to the west of the school consisting of insulation boards and roof membrane.  

Figure 6.  The Southgate elementary school viewed from the west (labeled 2a in figure 3).
The southwest side of the school featured a peculiar roof uplift forming an arch, upon which the roof soffit was tilted into the vertical and may have subsequently acted as a vertical load support (figure 7).  Whether this support prevented the collapse of the roof deck is possible though unknown at this point.

Figure 7.  The school viewed from the southwest (labeled 2b in figure 3).
 The interior view of the roof uplift revealed in figure 8 shows an unreinforced CMU block wall.  In this hall the weak link was the grouting one course below what appears to be a very thin bond beam.  In the next room to the east the bond beam separated from the steel web joists.  The interior also shows the soffit that tilted vertically and helped to possibly keep the roof supported through its studs and plywood sheathing.  It’s not known here whether the uplift pressure was from  Bernoulli forces above the roof edge or interior pressure from breached windows in adjacent classrooms.  More likely a combination of the two forces pushed the roof upward.  The roof covering was still intact in this picture and shows insulation boards attached to the web joists.  I was surprised to see no metal roof decking.  Thus I considered this roof to be lower than expected quality for a wind speed estimate.

Figure 8.  An interior view of the uplifted roof over the entrance visible in figure 7.
Two rooms to the northwest a classroom lost all the roof decking, most likely contributing to the debris field to the west of the school.  No metal roof decking was visible in the debris field (figure 9).  Though here the bond beam appears to be a metal girder, stronger than the masonry bond beam further southeast, I considered this roof to be lower than expected quality here as well due to the lack of significant roof decking.  I rated this portion of the school as uplift of roof decking and significant loss of roof covering (>20%) with an expected wind speed of 100 mph.  I consequently lowered the wind speed to 95 mph.  This estimate still yields an EF1 rating.

Figure 9.   An interior view of a class room on the west side of the school.
A second area just east of I-35 had a single story house that lost the entire roof (figure 10).  The team in which I was on inspected this house to determine which side of the EF1/EF2 boundary the house should be rated.  Every house in the expected damage track was rated but only those appearing with colored triangular icons exhibited damage.  Thus the vortex was mostly too weak to cause damage except to a few houses where a brief damaging subvortex may have occurred. 

Figure 10.  An overhead map of the area of interest near Broadway Ave. just east of I-35.  The inset left points to the same area as the white outlined box in the main map.  The circle in the inset refers to the house in question.

This image in figure 11 shows the roof debris blown to the south side of the house consisting of composite shingles, 1by4” decking boards (likely tongue and groove) and 2X4” rafters spaced 24” apart.  Note the adjacent house to the east exhibited no apparent damage.   Adjacent houses to the west and north also showed no visible damage.

Figure 11.  A view of house labeled photo 2 in figure 10 facing northwest.   Figures 12 and 13 were taken at the label '1a and 1b' in the photograph. 
 This image on the east side of the house shows the part of the gable wall top plate and 2 nails (6d?) separated every 18” approximately (figure 12). 

Figure 12. A view of the top end of the collapsed gable roof including the gable wall top plate.  

The rest of the gable showed two nails between each rafter and the gable top plate approximately every 18”  (figure 13).  The gable fell nearly intact, however the orientation of this side was nearly parallel to the strongest wind, as determined by the direction of debris. It appeared though that the gable was poorly connected and exhibited no bracing.  While the lack of bracing appears to be common, the lack of a strong connection also appeared to be less than typical quality construction.

Figure 13.  A view of the rest of the gable.

Perhaps more significant than the low construction quality of the deroofed house was the lack of damage outside that caused by the debris.  These surrounding houses to the southwest showed little to no damage and even the fences remained standing outside the debris zone (figure 14).  The backyard shed was tipped over but it appeared to have no connection to the foundation other than gravity.  The light poles also were undisturbed except where the wires intercepted blown debris.  The only damage in the immediate vicinity occurred from impacts from debris.  As the debris from the roof and a car port blew to the southeast, it struck the fences and another house to the southeast resulting in the loss of exterior brick fa├žade and several windows (figure 15).  
Figure 14.  A view to the southeast from the house.  

Figure 15.  Two panoramas where the top one faces the house from the south end of its backyard.  The bottom panorama was taken from the backyard to the southeast of the house labeled '2' in figure 10.

Why did only this house suffer a loss of roof with the surrounding houses showing negligible damage?  One possibility is that this is the only house to have a car port in the vicinity (figure 16).  So there are two possibilities that affect our decision-making:  1)  The car port debris weakened the roof upon impact causing its failure. 2) A subvortex was small enough to only affect this house to the exclusion of all others.   Both possibilities yield a lower than expected wind speed because either collateral damage could’ve lifted off the roof at relatively low wind speeds or the duration of the subvortex over any spot would've led to a less than 3 seconds.   Consider that the vortex signature on the WSR-88D was moving between 40 and 45 mph and its diameter of maximum winds would be only 50 ft, the maximum winds would not have lasted the standard three seconds defined in the EF-scale.   This vortex would’ve needed to have spun up immediately northwest of the house because there was no visible damage to the houses across the street to the northwest.  However one house to the southeast (not the one with collateral damage) suffered loss of roof deck panels, suggesting the subvortex continued on to the east-southeast.

The degree of damage was labeled as large sections of roof removed, walls remain standing but the three second wind speed was lowered to roughly 105 mph or EF1.  The most likely scenario of what happened here is relatively weak connections between the top wall plate and the roof, along with possible impact from the car port allowed the subvortex to remove the roof from the house.
Figure 16.  A pre- and post-tornado view of the deroofed house as viewed from the northeast.  The pre-tornado view courtesy of Google Streetmaps.
 The final damage indicator that straddled the EF1/2 boundary was in east Moore where a 3600 sq ft house was directly hit by the core of the tornado (figure 17). 

Figure 17.  An overhead view of the final area under consideration, ShadyCreek St. just north of 19th in east Moore.  The inset shows the photo locations in subsequent figures including a wide panorama.
The house is a large single story wood framed structure with a hip roof design, brick and stone veneer and a composition shingle roof on OSB decking.  All the walls were constructed with 2X6” pine studs and roof rafters.  The owner built the house in 2014 and applied updated buildings codes that he promoted to the Moore City council following the May 20, 2013 EF5 tornado.  He mentioned that the house was built with anchor bolts and metal clips from the sill plate up to the top plate. 

The house experienced significant loss (>20%) of roofing above the complex roof-to-wall connections including the decking and numerous rafters (figure 18 and 19).  However there was no damage to windows or the garage doors on the north side of the house.  The adjacent garage shop west of the house lost shingles (DOD=1) and underlay on the south side while the neighboring house lost some shingles (DOD=1) on its west side (figure 18).

Figure 18.  A view of the three buildings including the house with the damaged roof.  Two insets indicate a pre- and post tornado view of the house.  

Figure 19.  A zoomed in view of the west side of the house.

 The tornado passed through the shelterbelt west of the house felling two large multiple trunk Elm trees with trunk diameters approaching 2’ (figure 20).  The smaller trees on either side were not affected thus supporting the premise that larger trees suffer disproportionately from severe winds even in an unfoliated state.   The tree damage indicators in the EF-scale were difficult to apply when vastly different tree response occurs within a just a few meters.  I used the standard wind speed of 100 mph based on either an uprooted or snapped tree trunk and assumed the tornado was of similar strength as at the house.  A narrow section of felled trees also could be seen in the woods opposite the field to the northwest.  The narrow width of felled trees strongly supports the idea of a narrow vortex progressing across the field and over the most affected house.  Due to its small diameter the buildings on either side escaped with relatively little damage.

Figure 20.  A panorama (labeled 'pano' in figure 17) including the houses and the shelter belt to the west.
 With the rafters clipped to the top plate and the lack of damage in the lower roof, why was the upper roof as damaged as it was?  A closer inspection of the roof near the ridge board revealed that the OSB decking was stapled to the rafters at irregular intervals (figure 21).  We could not measure the interval between staples but they were almost certainly spaced at greater than 6” intervals.  Since the building envelope was not breached it’s likely external pressure forcing overwhelmed the limited attachments of the roof decking.  The standard composition shingles offered no more than standard resistance to the wind.  This is below the quality of typical construction to give an expected wind speed and thus the wind speed estimate required to do this damage fell short of an EF2 rating.   Had the decking been clipped to the rafters this roof could’ve survived and EF2 intact and thus similar damage would’ve resulted in a much larger wind speed estimate.  Upon talking to the owner at the time, he admitted that he paid a lot for this flaw in his design.  If he used the same materials and techniques used to construct the porch overhang in framing the roof, he may have saved himself a lot of grief (see figure 22). 

Figure 21.  A zoomed view of the roof ridgeline  labeled '3,4' in figure 17 showing staples used to fasten OSB roof decking to the rafters.  

Figure 22.  A view of the east side of the house.

This survey though is about more than a single rating for the tornado.  With the advent of the Damage Assessment Toolkit (or DAT), we were able to create a high resolution survey from which the whole track could be contoured by EF scale rating.  The map that Doug Speheger of the NWS office in OUN, and others, created from this survey easily shows how small an area the EF2 winds occupied compared to the whole area that the tornado covered (figure 23).   By far the majority of the area covered by this tornado was only EF0 in strength and thus the energy that this tornado exacted upon the landscape was not as big as its EF2 rating suggests it may be.  

This is where high resolution surveys provide their biggest benefit.  Now we can see what fraction of the tornado path is occupied by winds of various strengths instead of making assumptions that has been standard practice based on a small sample of  well surveyed tornadoes.  Now the NWS has the opportunity to vastly increase that sample size and possibility to eliminate the need for more assumptions.  This information is more useful to NWS partners than many of us think.   Considering the users that include insurance industry, the Nuclear Regulatory Commission, risk analysts, FEMA, state and local governments and more and you may begin to see the value that comes from this level of detail in this NWS product.  It doesn't take a team of 12 people for every 12 miles to create them either.  It can be streamlined to be done quickly.  I'll reserve another post for that.  In the meantime, keep them coming!
Figure 23.  The map of the tornado path contoured by EF scale  published by NWS Norman.


To clarify, the media didn't force a reassessment of the tornado rating. Rather the initial rating of the EF1 allowed the NWS time to make a better assessment while still giving the public a preliminary assessment. 

Sunday, December 28, 2014

Could the Norman, ok snow of 27 Dec 2014 have been anticipated?

I'm sure the heavy snow that began early in the  morning of December 27, 2014 in Oklahoma was a surprise to many; an event which could've led to more significant impacts than what we saw given that it occurred only 2 days after Christmas .

 Was the snow event really as surprising as implied, and if not was there a way to effectively communicate a seemingly low probability event like this? In this day of exceptional numerical weather prediction, we shouldn't have seen a dry forecast just 12 hours before 3-6" of snow fell along the I44 corridor.  So the question is, was there no indication that a snow storm was on its way during the previous evening?
A scene typical of central Oklahoma roads.   Unknown source. 

Storm total snow amounts courtesy of NWS Norman. 

When the snow began to fall in thin bands as the radar image showed it appeared that an elevated frontal circulation was possibly involved. The 700 mb frontogenesis analysis in the SPC meso analysis plot helps support the idea, with the best values nicely lined up parallel to the snow bands observed by radar. This is important because the model guidance should have a similar signature in the forecasts. 

Frontogenesis at 700 mb in purple contours at 18 z from SPC.  

The NAM forecast model, however, didn't show much of any precipitation from the previous evenings run. What it did show in the Texas panhandle, however, appeared to be aligned in a similar direction as the frontal bands observed the next morning. But the NAM was as dry over central Oklahoma as the forecasts. 

The NAM 15 hr  forecast valid Dec 27 of 3 hour precipitation courtesy College of Dupage

The GFS evening model run, however, showed a slightly wetter pattern as a band of three hour precipitation accumulation appeared in roughly the same orientation and placement as was observed. The amounts were not quite up to the observed values but it's orientation gave a clue that it was on the right track that something may have fallen out of the frontal circulation aloft and that possibly an inch of snow may fall by late morning the next day.  The only problem was that this GFS forecast was available only after the evening news cycle was completed, thereby limiting its usefulness for planning in the evening before the event. Was there information in the guidance that could've been made a bit earlier?
Same as above except for the GFS. 

Well, possibly. In the afternoon before the snow day the short range ensemble forecast system, or SREF, showed that a several of its 15 runs forecast over one inch of snow in a corridor quite similar to the bands observed the next day. The probability of at least one inch of snow was only up to 20% in the college of du page model output but the SPC plume diagram valid for Norman showed that the several runs depicting the snow were very similar in timing and amounts. At least this output should give one pause that something may happen. 

The SREF probability of 12 hour snow exceeding 1" valid 18z Dec 27 courtesy of the College of Dupage.  

An SREF plume diagram of snow fall for Norman, OK from 21z Dec 26 courtesy of SPC. 

By 9:30 pm the evening before, three successive runs of the HRRR (high resolution rapid refresh) model were available that showed an axis of snowfall accumulation approaching or exceeding 1" right over the I44 corridor into the OKC metro area.  The hourly model runs allows its users to assess the consistency of its forecast. When three runs in a row showed a similar forecast then the likelihood of accumulating snow may not have been so improbable. 

The HRRR total snowfall at forecast hour 15 for the 23 z Dec 26 and 01 z Dec 27 runs courtesy of NOAA/GSD. 

So the big question of the evening was whether or not the guidance provided enough confidence to forecasters to go with some kind of winter weather warning.  In the NWS, forecasters are provided with a set of guidelines on when to issue winter storm watches, warnings and winter weather advisories. The NWS could've issued a winter storm watch, however they would need a 50% chance that winter precipitation would actually fall and accumulate to create hazardous travel conditions (see the directives at However we could debate for a long time whether or not there was a 50% chance based on the available guidance. One forecaster may consider that there needs to be a 50% chance that a winter storm warning criterion snowfall occur while another may not. I believe no one would argue that the chances of 4" or more accumulation was nowhere near that 50% value. Yet the directives suggest that a local forecast office has great latitude in determining the criteria for issuing a winter storm watch. Having snow falling enough to cover roads at rush hour would certainly be a good reason to consider sliding the threshold downward.  But this is a judgement call and the uncertainty in the guidance is enough that I can understand a forecaster decision not to issue a winter storm watch.  

No watch was issued for this event, but perhaps because a forecaster considered the chances of winter weather to be at 20 or 30%. With those low, but not negligible odds, What kind of message did a forecaster have available to advise the public?  A hazardous weather outlook could've been issued. However those kinds of products are usually reserved in the day 3-7 lead times. A special weather statement could've been issued since its format provides enough flexibility to provide a variety of information.  Perhaps then the statement could've been backed up by reposting to newer avenues of communication such as online briefings, social media and nwschat.  Typically these avenues are reserved for winter events with a higher confidence forecast. 

Communicating a heads-up about low probability events is commonplace in convectively induced severe weather events but not for winter weather situations. Yet in the plains states, and other places, high impact events are often dominated by mesoscale processes (eg. Frontal bands, cold frontal squall lines), as this event demonstrated.  These events are going to be more difficult to forecast, and therefore, lower confidence of occurring as anticipated for any one spot, much like summer convection. Thus the communication required to warn for the potential impacts have to be different than the synoptically dominated snow storms for which the traditional avenues of communication are intended.  The challenge is when and what to communicate with these lower probability but potentially significant events.