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Jan 31 - Feb 1 Event - STORM MODE THREAD


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Updated LWX AFD:

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AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
900 PM EST FRI JAN 29 2021

.SYNOPSIS...
HIGH PRESSURE WILL BUILD OVERHEAD TODAY THROUGH SATURDAY. LOW 
PRESSURE WILL APPROACH FROM THE WEST SUNDAY AND TRANSFER OFF 
THE COAST ON MONDAY. HIGH PRESSURE WILL BUILD BACK IN BY THE 
MIDDLE OF THE WEEK.

&&

.NEAR TERM /THROUGH SATURDAY/...
9:00 PM UPDATE: NO SIGNIFICANT CHANGES TO THE FORECAST AT THIS
TIME ACROSS THE AREA. MAIN CHANGE FOR TONIGHT WAS TO DECREASE
WIND SPEEDS ACROSS THE AREA A LITTLE QUICKER, AND TAKE WIND
CHILL VALUES UP A FEW DEGREES OVERALL. OTHER THAN THAT, STILL
EXPECTING A DRY NIGTH UNDER MOSTLY CLEAR SKIES FOR MUCH OF THE
AREA. CLOUDS WILL LINGER ACROSS NORTHEAST MD AND FAR NORTHERN VA
FOR A WHILE LONGER THOUGH, BEFORE EVENTUALLY CLEARING OUT BY
DAYBREAK. SEE PREVIOUS NEAR TERM FOR MORE DETAILS ON THE
FORECAST THROUGH SATURDAY. 

PREVIOUS DISCUSSION: WINDS SHOULD GRADUALLY DECREASE IN 
MAGNITUDE OVERNIGHT AS HIGH PRESSURE STARTS TO APPROACH FROM THE
WEST, BUT THE COLD AIR WILL REMAIN IN PLACE. LOWS TONIGHT WILL 
BOTTOM OUT IN THE TEENS FOR MOST, WITH LOWER 20S POSSIBLE IN 
DOWNTOWN DC.

HIGH PRESSURE WILL BUILD OVERHEAD DURING THE DAY TOMORROW,
BRINGING QUIET WEATHER CONDITIONS TO THE AREA. SKIES WILL START
OUT SUNNY, BUT HIGH CLOUDS WILL BEGIN TO STREAM IN ALOFT IN
ADVANCE OF THE NEXT SYSTEM APPROACHING FROM THE WEST BY DAY'S
END. HIGHS TOMORROW WILL TOP OUT IN THE 30S TO NEAR 40.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY NIGHT/...
9:00 PM UPDATE: NO CHANGE TO THE PREVIOUS FORECAST AT THIS TIME,
BUT SOME INCREASE IN EXPECTED SNOW TOTALS MAY BE NECESSARY IN
THE NEXT SHIFT. MODEL GUIDANCE CONTINUE TO INCREASE IN
CONFIDENCE OF 4-7" FOR MOST AREAS ALONG AND WEST OF I-95, WITH
THE HIGHER END OF THAT STARTING TO LOOK A LITTLE MORE LIKELY.
WOULD LIKE TO SEE ANOTHER ROUND OF GUIDANCE BEFORE BUMPING
THINGS UP JUST YET. FOR NOW, SEE THE PREVIOUS UPDATE AND
DISCUSISON FOR MORE DETAILED THOUGHTS. 

7:30 PM UPDATE: ONLY CHANGE TO THE SHORT TERM FORECAST WAS A
SLIGHT UPTICK ACROSS WESTERN PORTIONS OF THE FORECAST AREA IN
TERMS OF SNOW TOTALS. THINK A STEADIER BAND OF SNOW MAY SLIDE IN
RATHER EARLY ON SUNDAY MORNING ACROSS THE EATERN WV AND WESTERN
MD. HAVE ADDED ALL REMAINING AREAS (WESTERN AND NORTHERN MD, AS
WELL AS THE REST OF EASTERN WV) TO THE WINTER STORM WATCH. SEE
THE LATES SNOWFALL FORECAST AT WEATHER.GOV/WASHINGTON/WINTER. 

PREVIOUS DISCUSSION: ATTENTION WILL TURN TO A SYSTEM TRACKING 
INTO THE OHIO VALLEY SATURDAY NIGHT. A SURGE OF WARM ADVECTION 
ALOFT IN ADVANCE OF THE SURFACE LOW/CLOSED UPPER LOW TRACKING 
INTO THE OHIO VALLEY WILL START TO OVERSPREAD THE AREA FROM 
SOUTHWEST TO NORTHEAST OVER THE COURSE OF THE NIGHTTIME HOURS. 
SNOW MAY BE POSSIBLE ACROSS CENTRAL VIRGINIA AND THE CENTRAL 
SHENANDOAH VALLEY AS EARLY AS MIDNIGHT, AND SHOULD REACH THE DC 
METRO BY AROUND OR SHORTLY BEFORE DAYBREAK. THE ONSET OF SNOW 
MAY HOLD OFF A COUPLE OF HOURS LATER IN THE BALTIMORE METRO 
AREA. A STEADY PERIOD OF SNOW IS EXPECTED ACROSS THE ENTIRE AREA
ON SUNDAY IN ASSOCIATION WITH THIS PUSH OF WARM ADVECTION. A 
GENERAL 4-6 INCHES OF SNOW IS EXPECTED ACROSS THE AREA WITH THIS
ROUND OF SNOW. AS A RESULT, A WINTER STORM WATCH HAS BEEN 
ISSUED FOR MOST OF THE FORECAST AREA.

MODEL GUIDANCE SHOWS A DRY SLOT MOVING IN ALOFT SUNDAY EVENING.
THIS DRY SLOT APPEARS AS THOUGH IT MAY SHUT OFF ICE CRYSTAL 
PRODUCTION FOR MUCH OF SUNDAY NIGHT, WITH SATURATION IN MODEL
SOUNDINGS EVIDENT ONLY UP TO BETWEEN -5 AND -10 C. WITHOUT
DEEPER SATURATION, PRECIPITATION SHOULD BE MUCH LIGHTER, AND MAY
FALL LARGELY IN THE FORM OF FREEZING DRIZZLE/LIGHT FREEZING 
RAIN, WITH LIGHT SNOW AND SLEET ALSO POSSIBLE IF ICE CRYSTALS 
ARE ABLE TO FORM. AS A RESULT, NOT MUCH ADDITIONAL SNOW 
ACCUMULATION IS EXPECTED SUNDAY NIGHT, ALTHOUGH THERE MAY BE A
LIGHT GLAZE OF ICE IN MANY AREAS. WARMER AIR MAY WORK INTO
SOUTHERN MARYLAND, WHERE A CHANGEOVER TO PLAIN RAIN WILL LIKELY 
OCCUR.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
BY MONDAY, THE UPPER TROUGH/LOW WILL BE MOVING OVERHEAD AND THE 
SURFACE LOW WILL TRANSFER TO THE COAST, LIKELY SOMEWHERE BETWEEN 
CAPE HATTERAS AND THE DELMARVA. AS COLDER AIR MOVES IN ALOFT 
ASSOCIATED WITH THE UPPER TROUGH AND MOISTURE DEEPENS ONCE AGAIN, 
PRECIPITATION TYPE WILL LIKELY TRANSITION TO SNOW, THOUGH SOME RAIN 
IS POSSIBLE DEPENDING ON HOW FAR WARM AIR WORKS INLAND. THIS PART OF 
THE FORECAST REMAINS TRICKY, BECAUSE ADDITIONAL SNOW ACCUMULATIONS 
WILL BE DEPENDENT ON THE EVOLUTION OF THE LOW AND MESOSCALE 
FEATURES. SO ADDITIONAL SNOW TOTALS COULD RANGE FROM BEING VERY 
MINIMAL TO HAVING A LOCALIZED ZONE OF ADDITIONAL MODERATE OR HEAVY 
SNOW. SNOW WILL LIKELY TAPER OFF MONDAY NIGHT AS THE LOW PULLS AWAY 
TO THE NORTHEAST. SOME PRECIPITATION MAY LINGER INTO TUESDAY ALONG I-
95 AS WELL AS UPSLOPE ZONES, BUT THE IMPACTFUL PRECIPITATION WILL BE 
OVER BY THEN. OTHERWISE, GUSTY NORTHWEST WINDS WILL DEVELOP AS THE 
LOW STRENGTHENS, AND TEMPERATURES WILL REMAIN CHILLY.

 

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1 minute ago, MN Transplant said:

Our worst-case scenario seems to be the 00z NAM Nest dry front side and the 00z 12km NAM dry backside.  That’s about a 3” storm for many of us along 95.

I think the backside will moisten up some.  At least I hope or it’s gonna be a painful process.  

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4 minutes ago, snowfan said:

It was dry for the most for us on the back end. Most of us get about .1 from hr60 to hr84.

We get the 700 wind vector pointed at us, but it is not a nice moist fetch off the ocean, it is a very long route over NYC and the coming in from the north.  Just not much left by the time it gets here.

06z Monday to 06z Tuesday.  Not much after either.

16F8708D-BF46-457E-A0AA-127F0362F3CF.png.3882d48ede6da98fb0b56a51bb7c3f7a.png

94EBB87E-A9A4-40B8-A7D4-906D7302D415.png.fb21ad72d2a78890008894e9edd09b97.png

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2 minutes ago, MN Transplant said:

Our worst-case scenario seems to be the 00z NAM Nest dry front side and the 00z 12km NAM dry backside.  That’s about a 3” storm for many of us along 95.

My amateur look at this evening's NAM prompts this question to a/any meteorologist:   Check out the 700 mb evolution:   700 meanders SW to NE at Ohio River valley and weakens... and then reforms in the mid-Atlantic states moving east and intensifying over eastern Delaware.   It warms and dry-slots us.   Are we s-------d or is NAM incorrect?

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NAM 500 trough is swinging across positively tilted and the upper low is closed, but the best forcing is generally north of where the closed circulation crosses. Trough needs to swing across neutral or even a bit negative tilt. Coastal does not get captured or tuck where it needs to and we torch, stay out of the CCB. 

E166091E-8A20-45A1-A13F-EEF62D3A8537.jpeg

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4 minutes ago, winterymix said:

My amateur look at this evening's NAM prompts this question to a/any meteorologist:   Check out the 700 mb evolution:   700 meanders SW to NE at Ohio River valley and weakens... and then reforms in the mid-Atlantic states moving east and intensifying over eastern Delaware.   It warms and dry-slots us.   Are we s-------d or is NAM incorrect?

Yeah, it’s a transfer.  You can see the 12z Euro does the same, just with a different, more southerly orientation that helps us out some.

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1 minute ago, Wonderdog said:

Just remember, it's a Miller B. 

That’s why I liked the euro runs with a weaker coastal that went E rather than NE up the coast from our latitude. Wasn’t a blockbuster but it was clean. 

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NAM 500 trough is swinging across positively tilted and the upper low is closed, but the best forcing is generally north of where the closed circulation crosses. Trough needs to swing across neutral or even a bit negative tilt. Coastal does not get captured or tuck where it needs to and we torch, stay out of the CCB. 
E166091E-8A20-45A1-A13F-EEF62D3A8537.thumb.jpeg.25575d5c2aa757c4c273a209a71eead7.jpeg

At least it’s just the nam


.
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There will probably be some drizzle and freezing drizzle once the dry slot rolls in. Could be an extended period of it too. The reason is that cloud top temps warm above -10C as the dry slot erodes it away, leaving no very few ice crystals available for snow formation. Still plenty of moisture underneath that and some background lift though, so there will be drizzle. That's going to be an issue until/unless the coastal low can set up a reasonable def. zone and ccb to remoisten the column.

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Just now, Eskimo Joe said:

IMO, LWX's new snow map is decent but given the NAMs being bad for us, I don't see it going any higher.

I was just thinking that it’s interesting when the pros see things one way and weenies see it another. I can’t say for certain but I highly doubt they are hanging their forecast on the NAMfrom this range

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3 minutes ago, DCAlexandria said:


At least it’s just the nam


.

Attention should be paid to it. The NAM even beyond 48 hours can give insight into how the smaller details may be evolving. Guidance is guidance. We shouldn’t look at it’s QPF verbatim, but more so to understand it’s evolution.

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Just now, WinterWxLuvr said:

I was just thinking that it’s interesting when the pros see things one way and weenies see it another. I can’t say for certain but I highly doubt they are hanging their forecast on the NAMfrom this range

This. 

I'm sure Lwx isn't second guessing there snow map because of a NAM run from 48 hours out. Lol.

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Just now, ers-wxman1 said:

Attention should be paid to it. The NAM even beyond 48 hours can give insight into how the smaller details may be evolving. Guidance is guidance. We shouldn’t look at it’s QPF verbatim, but more so to understand it’s evolution.

       Correct.    Everyone laughed back in December when the NAM was the first model to show that storm coming much further west and ruining chances for heavy snow along the I-95 corridor.     That doesn't at all mean that it's correct now, but to completely dismiss it at this stage comes with risk.

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6 minutes ago, high risk said:

       Correct.    Everyone laughed back in December when the NAM was the first model to show that storm coming much further west and ruining chances for heavy snow along the I-95 corridor.     That doesn't at all mean that it's correct now, but to completely dismiss it at this stage comes with risk.

You and I are on the same page, NAM kept bumping that mix line NW and folks just laughed it off until it became clear things were going that way. Also wanted to say that somehow when I quoted this post it added a ''laugh'' reaction and I can't change it because I hit my limit for the day. Weird.

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10 minutes ago, high risk said:

       Correct.    Everyone laughed back in December when the NAM was the first model to show that storm coming much further west and ruining chances for heavy snow along the I-95 corridor.     That doesn't at all mean that it's correct now, but to completely dismiss it at this stage comes with risk.

What was odd was the mslp actually improved slightly but the storm had a “blown out” look to it and never wrapped back up. Very odd to have a captured tucked low off OC and all the precip is up in PA.  

ETA: ehh on second look the low gets captured further north then I thought on first glacé. 

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