Major Snowstorm looming

1:00PM UPDATE:
Snow totals are in...

...MONTGOMERY COUNTY...
   3 NNW GERMANTOWN      19.0  1031 AM  2/13  NWS EMPLOYEE            
   DAMASCUS              19.0  1030 AM  2/13  NOW GLAZE OF ICE ON TOP 
   1 ESE DAMASCUS        17.0  1000 AM  2/13  PUBLIC                  
   2 NNW BOYDS           16.2  1111 AM  2/13  PUBLIC                  
   1 NNE WASHINGTON GRO  15.0  1100 AM  2/13  TRAINED SPOTTER         
   1 ESE NORBECK         14.4  1048 AM  2/13  TRAINED SPOTTER         
   1 WNW OLNEY           13.9   943 AM  2/13  TRAINED SPOTTER         
   N ROCKVILLE           12.5  1023 AM  2/13  NWS EMPLOYEE            
   2 SSW ROCKVILLE       11.0  1024 AM  2/13  TRAINED SPOTTER         
   1 ESE TAKOMA PARK      9.2  1000 AM  2/13  TRAINED SPOTTER         

7:00AM UPDATE:
Widespread totals of in excess of a foot throughout the area due to persistent mesoscale banding over the region last night resulting in heavy snowfall rates of 1-3 inches/hr, The snow/sleet line has worked its way into DC and southern Moco. How far west it goes before retreating has yet to be seen. It is likely that it will not move past southern Moco and begin to retreat in the coming hours.

Blue is rain sleet line. Green is sleet/rain line possibly.

3:30AM UPDATE:
The changeover to sleet is beginning. some spots in DC are reporting sleet though this could be h=just an isolated occurrence. In addition, the NWS has extended the winter storm warning due to the possibility of heavy snow as the backside of the storm swings through thursday evening.

1:20AM UPDATE:
The NWS has issued a special weather statement for the DC area for snowfall rates up to 3 inches an hour associated with a heavy snow band in the area.

...HEAVY SNOWFALL EARLY THIS MORNING...

A BAND OF HEAVY SNOW OVER THE VIRGINIA PIEDMONT IS EXPECTED TO
OVERSPREAD THE WASHINGTON METROPOLITAN AREA BY 2 AM THIS MORNING.
THE HEAVY SNOW WILL OVERSPREAD THE BALTIMORE METROPOLITAN AREA
BETWEEN 2 AM AND 3 AM THIS MORNING.

HEAVY SNOW WILL PERSIST FOR A COUPLE HOURS WITH VISIBILITY REDUCED
BELOW ONE-QUARTER MILE. SNOWFALL RATES OF ONE TO AS MUCH AS THREE
INCHES PER HOUR ARE EXPECTED UNDERNEATH THIS BAND. TRAVELLING WILL
BE VERY DANGEROUS DURING THIS TIME SINCE ROADS WILL BECOME QUICKLY
SNOW-COVERED.

In addition, it has raised snowfall totals in the warnings from 6-10 to 7-12. We could be looking at an over performing storm. There is a possibility of the rain during the daytime changing back to snow thursday night.

...WINTER STORM WARNING REMAINS IN EFFECT UNTIL 5 AM EST FRIDAY...

* PRECIPITATION TYPE...HEAVY SNOW.

* ACCUMULATIONS...7 TO 12 INCHES...WITH UP TO 15 INCHES AT
  ELEVATIONS ABOVE 800 FEET.

1:00AM UPDATE:
The models are right on the money. Giant snow band is heading for DC with 1-2 inch+ snowfall rates. It just keeps building back south. This is going to be fun to watch.

Radar @ 1:00 snow huge 35-50 dbz snow band to the south of DC heading northeast

10:40 PM UPDATE:
The hi-res models have come in with INSANE amounts of snow. For example, the RAP gives us a foot of snow through 10:00AM and the HRRR also gives us a foot through 9:00AM. These snow totals do not include compaction or any melting that may occur so the actual snow depth will be lower. However, these models also indicate high instability which is responsible for these snowfall rates. Richmond, Va has already reported thundersnow. Just for fun, I will issue a Thundersnow Watch (not something that actually exists) for Moco from 12:00 AM through 6:00AM. Although this does not guarantee thundersnow, this does mean that the potential for it is there.

8:15 UPDATE:
Light snow has broken out across the region and will increase in intensity throughout the night. Cold temps will make sure the snow accumulates immediately. The SPC has issued a mesoscale discussion about the possibility of heavy snow.

AREAS AFFECTED...PARTS OF THE MID-ATLANTIC STATES

   CONCERNING...HEAVY SNOW 

   VALID 122341Z - 130515Z

   SUMMARY...A LARGE SWATH OF MODERATE TO HEAVY SNOW WILL DEVELOP NEWD
   THIS EVENING WITH RATES OF 1-2 IN/HR. THE MOST PERSISTENT BAND
   SHOULD EXIST ALONG THE BLUE RIDGE MOUNTAINS AND ADJACENT PIEDMONT
   FROM NWRN NC TO CNTRL VA THROUGH ABOUT 03Z (10:00PM EST) AND INTO THE WASHINGTON
   DC METRO AREA BY 06Z (1:00 AM EST).

   DISCUSSION...23Z SURFACE OBSERVATIONS CONFIRM A BROAD AREA OF
   MODERATE TO HEAVY SNOW AS FAR N AS THE I-64 CORRIDOR IN VA/WV. 12Z
   CONVECTION-ALLOWING WRF GUIDANCE GENERALLY APPEARS TOO SLOW WITH NRN
   EXTENT /OUTSIDE OF THE NSSL-WRF/ COMPARED TO RADAR/SURFACE
   OBSERVATIONS. CONSENSUS OF CONVECTIVELY-PARAMETERIZED GUIDANCE
   APPEAR CLOSER TO REALITY...WITH 15Z SREF MEAN AND 18Z NAM SUGGESTIVE
   OF SNOWFALL RATES AVERAGING 1-2 IN/HR. THIS APPEARS REASONABLE GIVEN
   INTENSE FRONTOGENESIS/ISENTROPIC LIFT DOWNSTREAM OF A VIGOROUS AND
   STILL AMPLIFYING SHORTWAVE TROUGH OVER THE SOUTHEAST. THE MOST
   PERSISTENT BAND OF HEAVY SNOW WILL LIKELY EMANATE NEWD FROM THE NWRN
   NC/SWRN VA AREA TOWARDS THE WASHINGTON DC METRO AREA BETWEEN 03-06Z.

ORIGINAL TEXT:
Let me put it simply. A major storm is now inevitable expect 4-8 inches of snow by sunrise, possibly more. snowfall rates will be in excess of 1 inch per hour for much of the night as the storm blows up off the NC coast.

IR satellite

Radar @ 7:15 PM 2/12/14

Tonight's forecast is final. temps in the mid to upper 20s and heavy snow. 4-8 inches expected.
The heaviest snow should be in the early morning hours. Snowfall rates then could approach 2 inches/hr and even 3 inches/hr in the heaviest convective bands. Thundersnow is definitely possible in the heaviest of snows. In addition, winds will pick up to gust near 25 mph at times.

HRRR 1hr snowfall from 3:00AM to 4:00AM  shows widespread heavy 1 inch per hour rates.

HRRR simulated radar, precip type and freezing line

 Now that we are in the short term, I have turned to high resolution models such as the HRRR model, which can do better to sniff out convective bands and other small scale features. The HRR shows heavy snow for DC throughout the night and into the morning. Although this may be overdone by 2-4 inches, it is possible. The HRR shows around 10 inches of snow by 6:00 AM

HRRR snowfall by 6:00 AM Thurs morning.

Tomorrow during the day is less clear. When the changeover to rain is the issue. The changeover near DC is expected around mid morning just as the heaviest precip heads east, dryslotting us. Places to the north and west may not change over at all while places to the south and east will change earlier. Light rain or rain snow mix is then likely for the rest of the day with temps in the low to mid 30s. In the afternoon, a second area of snow might swing through, mixing with rain and snow around the area. Depending on the strength of this "deformation zone", We could get another few inches of snow with the highest amounts, as usual, to the north and west. When all is set and done, we will range from 7 to 14 inches of heavy, wet, snow across the county.

DC is in the dry slot with snow to the west at 10AM

Temperatures are just above freezing. at 10 AM

Maryland is under a state of emergency and Montgomery County Schools have closed for Thursday. I will continue to have updates throughout the night. Any snowfall reports are appreciated and can be placed in the comments section.

Thanks for reading and enjoy the snow!

-Alex 

Possible Major Storm

10:45PM Update:
I just noticed something interesting, the temps stay in the low to mid 20s throughout the storm according to the NAM only to rise above freezing after the storm passes. These lower temperature would promote higher snow ratios up to 15:1 at the start to 10:1 near the end. The NWS also agrees:

00Z NAM PUTS LOW PRES IN A PRIME [POSITION] FOR SNOW PRODUCTION IN THE 
MID ATLC. 850-1000 THCKNS IN NAM GO THRU 12Z THU...AND ARE SUB 
1300 [ACROSS] THE CWA W/ THE XCPTN OF LWR SRN MD.

10:25 PM Update:
The GFS should be coming out soon and also, I got some pretty NAM maps for you. :)

10:1 ratio, likely not accurate

Gives DC an inch of liquid equivalent precip so depending on the temps and snow to liquid ratio, snow totals could vary with higher ratios to the west where it is colder and lower ratios to the east where it is warmer.

10:00PM Update:

The 00z NAM has come out and has a mixed bag. It has come back to earth with less insane snow totals, but it has the low going east, which cuts down even more on precip, with only 10-11 inches for the DC metro with less going North and West. It also delays the timing to early Thursday morning. But being a short range model, such tiny differences should not be worried about. Let's wait for the GFS and euro to come out to see if there are any clear trends. Remember, the models will keep flipping back and forth, so the snow amounts will not be clear until about 24 hours before the storm (we are about 48 hours out) and not certain until after the storm is over.

ORIGINAL TEXT:

The NWS has issued a Winter Storm Watch for its whole forecasting region for an excess of 5 inches possible wednesday night through friday.

THE NATIONAL WEATHER SERVICE IN BALTIMORE MD/WASHINGTON HAS
ISSUED A WINTER STORM WATCH...WHICH IS IN EFFECT FROM WEDNESDAY
EVENING THROUGH THURSDAY EVENING.

* PRECIPITATION TYPE...SNOW AND SLEET

* ACCUMULATIONS...THE POTENTIAL FOR 5 OR MORE INCHES OF SNOW AND
  SLEET.

* TIMING...SNOW IS EXPECTED TO MOVE IN FROM THE SOUTH WEDNESDAY
  EVENING. SNOW MAY MIX WITH SLEET AND RAIN LATE WEDNESDAY NIGHT
  AND THURSDAY. THE SNOW COULD BE HEAVY AT TIMES WEDNESDAY NIGHT
  AND THURSDAY.

* TEMPERATURES...IN THE MID TO UPPER 20S 20S WEDNESDAY NIGHT...
  SLOWLY RISING INTO THE LOWER AND MIDDLE 30S THURSDAY AFTERNOON.

* WINDS...NORTH 15 TO 25 MPH WITH GUSTS UP TO 35 MPH.

* IMPACTS...ROADS MAY BECOME SNOW AND SLEET COVERED AND SLIPPERY.
  TRAVEL MAY BE DANGEROUS WEDNESDAY NIGHT AND THURSDAY. HEAVY WET
  SNOW COULD LEAD TO SOME POWER OUTAGES.

The 18z models are out I will not be including the snow maps for the sake of time:

               North/West of DC  South/East of DC                          Notes:

GFS 18z            3-5                     6-10               Weakest solution, more out to sea

NAM  18z        10-20                 20-30            Insane, too wet, ideal low track, inaccurate at this range.

CMC 12z         10-14                 10-14             DC bullseye, weaker low track close to the coast

ECMWF 12z    14-24                 6-14              Shows freezing line west of I-95, low tracks                                                                                            farthest west bringing in warm air, strongest low

I would give a preliminary forecast of 6-10 inches of snow. Temperatures will be an issue and the snow could possibly mix with and change to sleet and rain as the low makes its closest approach. The disturbances that are responsible for the storm have not been fully sampled for data yet because it was out at sea for the 12z and 18z runs. The first fully sampled run will be tonight's 00z run which will come out in the 10:00-11:00PM timeframe. I will probably have an update then.

 Thanks for reading!

-Alex

Possible snow?

11:00PM update:
The 00z NAM came out wetter and farther west with the rain snow line over us, but the freezing line to the east, meaning a major ice storm. It is useless to worry about the rain snow line at the moment though. The line will shift around in the coming days. The simulated radar is quite impressive though.

The GFS gives us nothing again. Why is it so insistent on forcing it out to sea?

It would be interesting if the GFS turned out to be right...

ORIGINAL TEXT:
We got a little snow tease today. About 1/2 an inch of snow fell across the region. The is what is left of what had the potential to be a big snowstorm a week out but alas, the timing was off and it never got started. Next week, another storm is forecasted by models to have the potential to impact the Mid Atlantics somewhere around the wed/thurs timeframe.  As usual, the models have had issues with resolving the different pieces of energy that may come together to form the storm. There are a host of issues this storm has to overcome. The energy that  will hopefully come together to form the storm are still over the Pacific ocean. since this is a place where there is very little data collection, leading to initialization errors, which will add up.  Second, timing and temperatures will be an issue, as they have been all winter. The arctic high that is pumping cold air will be retreating east as the storm approaches. If the storm comes late, the high might warm us above freezing and we will get rain. The final issue is storm track. If the storm tracks too inland as the 0z Euro advertised, then we will get rain as warm maritime air wraps around the storm. If the storm tracks too far east like the GFS is saying, then we will get nothing. any model runs should be taken as guidance. Nothing will happen exactly as the models show.

Snow maps are calculated with 10:1 ratios. Actual ratios may be lower due to temperatures and melting. Any snow that falls will be wet and heavy.

ECMWF(Euro)
The Euro has been amazingly consistent with giving us a snowstorm even from a week out.

The 00z Euro tracked the storm further inland than previous runs, which would give us a couple of inches of snow before turning over to rain. This inland track is quite unlikely since initially there will be 2 lows, one in the gulf and one off the florida atlantic coast, that are fighting to gain dominance. Only if the low in the gulf low assumes dominance quickly will the storm track this far west.

The more recent run, the 12Z puts us in the bulls eye, with an unusually strong 988 Mb storm off the coast. The 12z Euro depicts an all snow storm with liquid equivalent precipitation in excess of 1 inch. This is the reasonable "perfect" scenario. 

The 12z ensemble mean lends strong support for the operational run that gave us the "perfect" snowstorm, with a swath of heavy snow up the east coast. The ensemble runs are low resolution runs using slightly different data. This attempts to account for possible data errors. The mean takes the average conditions produced by all the ensembles. If it matches or is similar to the the operational run, then the event is more likely to happen than if the operational run varies widely from the ensemble. The wider the variation, the less confidence the the run there is.

NAM

The NAM ended while the storm was still impacting the region but it draws in warm air during the storm, possibly inducing a changeover to sleet or rain, which cuts down on snow totals in the DC metro and points east while dumping on the areas west. The low in this case hugs the coastline. This warmer scenario is the most likely seeing that temps have been a problem all winter, when major precipitation events are in the vicinity. One thing I have against the NAM is that it is a short term, high resolution model and it tends to get very inaccurate beyond about 48 hours. This storm is at the end of its run, at 84 hours.

CMC
If the Euro is perfection, the CMC is the worst case scenario. ever. It bombs the low off of NC and brings it up the coast to Ocean city as a 981 Mb low then brings it father up to New Jersey as a 969 Mb low and finally across the Canadian Maritimes as an 961 Mb low. This is an absurd strength for a cyclone at this latitude. This is only Due to the strength of the low and the track off the coast, temps will not be an issue although the a shift east will mean we are left out of the heaviest snow. To give it credit, it forecasted the big rainout last week while the other models were screaming snow plus most of the other snow events. The 12z CMC run can be summarized as "high off crack" but it shows that the ingredients are there for an amazingly strong storm if the timing is right.




GFS
The GFS is the outlier here, bringing the out to sea. The 18z run was an improvement, giving us 3 inches using the 10:1 ratio (too high). The 18z and 06z runs are done with less data than the 12z and 00z, meaning that they will be less accurate.  However, there were no major differences between the 12z and 18z runs. The gulf low is not strong enough and is drawn out to sea by the FL low and give us close to nothing. The GFS has been very inconsistent this winter and quite inaccurate with mid to long term forecasts. At least it has backed off the notion of a big rainy noreaster.


Summary:
There is a potential for a major snowstorm but at 4 days out, it is impossible to know what will actually happen. The storm's track, strength and precip type will be affected where the arctic high is, the speed of the system and whether it can get organized fast enough. To get the snowstorm, we want the storm to organize quickly, move in while the cold air is still in place, and stay long enough to drop copious amounts of snow. I recognize that this is a tall order here in the Mid-Atlantic. Hopefully the GFS will join the EURO/NAM/CMC camp and forecast some more snow.

Thanks for reading!

-Alex