December Discobs Thread 2018

[yoda]

39 minutes ago, high risk said:

      There definitely does appear to be a window of opportunity west and northwest of DC Friday afternoon.   Instability is limited, but there is some cape, and lapse rates will steepen as the colder temps aloft arrive.     The shear is outstanding, and there will be forcing along the arriving front.

@Eskimo Joe

Sounds good... LWX in their AFD mentioned a chance as well

[Fozz]

59 minutes ago, donsutherland1 said:

That's one-tail. Keeping in mind that one can also get 3.8 sigma departures on the dry side, the probability of a 3.8 sigma departure is twice as frequent. If one is just measuring the probability of a 3.8 sigma departure above normal, then that's correct.

You're right, but I was talking about only the wet tail of the curve since that's where we are at.

[Roger Smith]

There was a storm similar to this around Christmas 1979, rainfalls of 2-3" fairly widespread in the eastern U.S. and Canada, and January 1980 stayed dry and mild. The biggest weather event of that winter was a blizzard in southeast Virginia on leap year day (Feb 29-Mar 1 1980). 

[high risk]

NAM nest shows a dramatic warm frontal passage, with low 40s for DC metro at 8pm and near 60 by midnight.

[yoda]

LWX afternoon AFD:

.NEAR TERM /THROUGH TONIGHT/...

Moderate to heavy rains still on track for tonight as warm air
advection pattern strengthens in response to deepening
low over the Appalachians. Latest 12Z ECMWF now aims 850mb
speed max of 75kt more over our area tonight rather than over
the Eastern Shore of MD. This likely increases threat for both
heavy rainfall/flooding and severe wx threat overnight. NAM and
RAP Bufkit soundings show increasing potential for wind gusts of
40kt to mix down for a brief period tonight around 06Z Fri.
Rainfall amounts of 1-2 inches are expected which may cause
flooding and perhaps flash flooding especially how soggy the
soils are.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...

Mid-level dry slot still fcst to come in between 12Z-15Z Fri
which would end the heavy rainfall threat. However, guidance
continue to show potential for a second narrow squall line to
move through the area late Fri and Fri evening with secondary
low and upper trough. Total lightning flash density products
from the 12Z ECMWF show increasing odds for charge separation
and thus lightning production with this second line. Also,
colder air will start filtering in Fri night with rain showers
turning to snow showers over the mtns. Upper trough axis finally
crosses the area late Fri night with precip shutting off east of
the mtns, but still gusty NW post-frontal winds lasting through
Saturday. Finally, improving weather Sat night with diminishing
winds.

[87storms]

30 minutes ago, high risk said:

NAM nest shows a dramatic warm frontal passage, with low 40s for DC metro at 8pm and near 60 by midnight.

i hope so.  that sounds awesome.  today was cooler than i expected.

also, radar looks real good...would have been a mecs/hecs for this region.

[WxUSAF]

1 hour ago, high risk said:

NAM nest shows a dramatic warm frontal passage, with low 40s for DC metro at 8pm and near 60 by midnight.

Cold rain and midnight highs. Playing to our wintertime strengths.

[jewell2188]

With this next round of rain already occurring, it really is amazing to think about the amount of rain we have had this year. Flooding is not fun for anyone but it truly is crazy to see another storm a week apart from each other deliver the same results and over and over again since spring. 

[das]

3 hours ago, peribonca said:

Really impressive slug of rain moving up the Eastern US. Don't think I remember such a wide expanse without any frozen even up to Hudson's Bay

The areal coverage of the warm advection precip is just staggering.  And, it is still expanding.  What a beautiful storm.

[Maestrobjwa]

3 hours ago, Fozz said:

I just plugged in that standard deviation, and if it reaches 3.8σ (assuming a normal distribution), then this year's precip total would be a 1 in 13000 year anomaly.

Wait, say WHAT??!! 

[Fozz]

1 hour ago, Maestrobjwa said:

Wait, say WHAT??!! 

Yup. If we assume a normal distribution of precip totals (which I haven't checked, but I'll probably plug it into Python when I get a chance), then that is the probability of getting a precip total that extreme on the high end (3.8σ to be exact). It's an incredible deviation from the average. And 120+ years is a decent sample size too, so I think it's at least pretty close to a normal distribution.

[yoda]

@high risk

From the updated evening AFD from LWX:

Quote

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... Latest surface analysis indicates low pressure currently centered near the southside Virginia/West Virginia border. It will be tracking across the forecast area tonight. Although rainfall appears to be light to moderate on radar, precipitable water of 1.3 inches per 00z LWX RAOB on saturated soils mean flooding not far away. Already received a report from Nelson County...the first of likely many. Flood Watch in effect areawide.

The 00Z RAOB depicts a sharp low level inversion at 950 mb. Winds above the inversion already at 50 kt. The challenge will be whether winds aloft can punch through this stable layer. Temperatures will be on the rise overnight (near 60 degrees DC south/east by dawn). Shear won`t be a problem. Some meso guidance solution do suggest that a couple hundred joules CAPE possible, especially in the 03z-09z window. This will be the same time that 850 mb winds forecast to peak at 75-80 kt. That`s too much wind in the lower layers (aligned with strong forcing) to ignore. Southern Maryland would be at greatest risk. &&

 

[mattie g]

5 hours ago, Roger Smith said:

There was a storm similar to this around Christmas 1979, rainfalls of 2-3" fairly widespread in the eastern U.S. and Canada, and January 1980 stayed dry and mild. The biggest weather event of that winter was a blizzard in southeast Virginia on leap year day (Feb 29-Mar 1 1980). 

Great...yet another data point for Maestro to sh*t his breeches about.

[Maestrobjwa]

10 minutes ago, mattie g said:

Great...yet another data point for Maestro to sh*t his breeches about.

I'm not saying a word about it...this time, YOU are the one that brought it up. Now a mod asked me to lay off...but if yall keep trolling me...kinda hard to do that! Not cluttering this thread again. I'm past that.

[Maestrobjwa]

5 hours ago, Roger Smith said:

There was a storm similar to this around Christmas 1979, rainfalls of 2-3" fairly widespread in the eastern U.S. and Canada, and January 1980 stayed dry and mild. The biggest weather event of that winter was a blizzard in southeast Virginia on leap year day (Feb 29-Mar 1 1980). 

Now, to actually add an observation...you said that January was mild? Well, I dont know about the dry part...I think...unless the LR is completely wrong...we might be able to cross the mild temps off the list, at least...lol

[yoda]

Uh... TVS sig in E Loudoun county on radarscope...

Now by Chantilly...

ETA: both gone now

[high risk]

56 minutes ago, yoda said:

@high risk

From the updated evening AFD from LWX:

 

          Definitely think we have some thunder potential overnight, but I haven't seen any forecast sounding with surface-based instability.      So, while there will be strong winds just above the surface, it seems like the chances of them mixing down are low.    I'm still more intrigued by the late afternoon/early evening time period Friday when we'll have modest but surface-based instability.  

[PrinceFrederickWx]

0.73” for the day so far. My temperature jumped up to 66 pretty quick. Windy out too.

[wxmeddler]

23 minutes ago, high risk said:

          Definitely think we have some thunder potential overnight, but I haven't seen any forecast sounding with surface-based instability.      So, while there will be strong winds just above the surface, it seems like the chances of them mixing down are low.    I'm still more intrigued by the late afternoon/early evening time period Friday when we'll have modest but surface-based instability.  

DCA TAMDAR aircraft at 2:58z had 50 kts at ~950mb (1900ft). Peak of LLJ at ~912mb (2800ft) with 63 kts. Nearly isothermal from SFC to that height.

[high risk]

24 minutes ago, wxmeddler said:

DCA TAMDAR aircraft at 2:58z had 50 kts at ~950mb (1900ft). Peak of LLJ at ~912mb (2800ft) with 63 kts. Nearly isothermal from SFC to that height.

       The isothermal profile surprises me, as the models have shown a pretty significant low-level inversion.    That could certainly up the threat of stronger overnight surface gusts.

        11pm:   60 at DCA, 57 at BWI, and 55 at IAD.

   

[Roger Smith]

On the 1980 analogy, it wasn't really meant as a full winter analogue, I just remembered this similar heavy rain producing low without snow in the circulation, then that's what followed. 

I do think there could be some very mild weather at times this coming 2-3 weeks followed by a turn to much colder around mid-January. Some of the patterns in the other thread discussion that look like they might bring cold are just long wave shallow troughs collecting Pacific air masses and would remain above normal even if upper level flow is somewhat depressed. That west coast ridge needs to amplify or move further east and buckle to get cold air moving south again like it was in November. 

On the severe risk, my guess is that this system will race northeast overnight with a triple point forming near BWI to central Chesapeake Bay in the late overnight hours, there could be some brief TRW- activity and gusts to 45 mph, but the more potent severe risk is probably going to be later Friday as this triple point moves through extreme e/ne NJ, se NY and CT. As the high amplitude trough pinches and hangback low pressure tracks over the DC region later Friday, some hail showers may develop, temperatures are probably going to soar overnight to low-mid 60s then fall back gradually to around 50 mid-day Friday at which time convection will produce some local hail. Rain will turn to heavy wet snow over highest portions of the nearby mountains. 

[wxmeddler]

Just now, high risk said:

       The isothermal profile surprises me, as the models have shown a pretty significant low-level inversion.    That could certainly up the threat of stronger overnight surface gusts.

        11pm:   60 at DCA, 57 at BWI, and 55 at IAD.

   

Yeah, DCA's winds just bumped up with that temp jump. There seems to be a bit more momentum transfer / mixing than expected. Only thing left not fully feeling it is Fredrick with 41 and NE'erlies still. 

[Maestrobjwa]

Just now, wxmeddler said:

Yeah, DCA's winds just bumped up with that temp jump. There seems to be a bit more momentum transfer / mixing than expected. Only thing left not fully feeling it is Fredrick with 41 and NE'erlies still. 

Sounds like we gonna have some runaway inflatable Santas...lol

[high risk]

Seems like a pretty good shot that northern VA, eastern WV, western MD, and south-central PA will get a MRGL risk for severe Friday, with an outside chance of an eventual SLGT.

[EastCoast NPZ]

38f here.  Hopefully that helps us avoid that damn wind.

[WEATHER53]

Gusty southerly breezes and up to 56F from 42 earlier 

[yoda]

4 minutes ago, high risk said:

Seems like a pretty good shot that northern VA, eastern WV, western MD, and south-central PA will get a MRGL risk for severe Friday, with an outside chance of an eventual SLGT.

For isolated damaging wind gusts or isolated tornado?

[Woodbridge02]

59.9 with fog, temperature rose from 48 in the last hour.

[Fozz]

Temp shot up from 38 at 4pm to 58 at 11pm.

[coolio]

Rain seems to be lighter and widespread.  When a yellow spot pops up, it quickly goes away.

Early models had a lot of training that were on or parallel to 95.  Does not seem to be doing that thus far.