Jan 25th Winter Weather Discussion Thread

[NGaWxNerd]

RGEM has Brick in the sweet spot

Lord help us one and all.

[HurricaneTracker]

Where is the moisture at??  I look on the national radar and there is NOTHING for what looks to be 100's of miles from here??  Is something just going to "magically" appear overnight??

 

Yes, and it's not magic.  It's QG Theory.

[packbacker]

Where is the moisture at?? I look on the national radar and there is NOTHING for what looks to be 100's of miles from here?? Is something just going to "magically" appear overnight??

Yes, NAM current sim radar...

 

 

NAM at 10am EST

 

 

[eyewall]

GFS is rolling now.

[MADDOGG]

WINTER WEATHER POSSIBLE FRIDAY ACROSS NORTHERN AND PORTIONS OF
CENTRAL GEORGIA...

/ISSUED 319 PM EST THU JAN 24 2013/
MAIN CONCERN FOR THE SHORT TERM IS WINTER WEATHER ON FRIDAY.  ALL
GUIDANCE POINTS TO THE DEVELOPMENT OF A CAD ACROSS NORTHERN GEORGIA.
THIS WILL AID IN DROPPING TEMPERATURES ACROSS THE AREA OVERNIGHT
TONIGHT.  AT THE SAME TIME...A COLD FRONT IS EXPECTED TO DRIFT SOUTH
ACROSS THE AREA...PROVIDING MOISTURE AND LIFT.  AFTER 12Z ON
FRIDAY...PRECIPITATION SHOULD BEGIN ACROSS THE AREA.  MOST OF THE
PRECIP WILL START AS FREEZING PRECIPITATION UNTIL SURFACE
TEMPERATURES WARM.  FOR MOST OF THE AREA...THIS FREEZING RAIN SHOULD
TURN TO RAIN BEFORE 18Z.  ACROSS THE NE MOUNTAINS.  THE WEDGE IS
EXPECTED TO HOLD ON LONGER.  THIS SHOULD LEAD TO A LONGER DURATION
FREEZING RAIN EVENT.  AS SUCH...TOTAL STORM ICE ACCUMULATIONS IN NE
GA WILL BE CLOSE TO A QUARTER OF AN INCH TO SLIGHTLY HIGHER.

WITH THAT ALL BEING SAID THERE ARE A COUPLE OF THINGS THAT STILL
CONCERN ME ABOUT THIS FORECAST.
1. WE HAVE VERY LOW DEWPOINT TEMPERATURES ACROSS NORTHERN GEORGIA.
MODEL SOUNDINGS DEPICT A SHALLOW DRY LAYER AT THE SURFACE.  AS
PRECIPITATION FALLS INTO THIS LAYER...THIS LAYER COULD COOL FASTER
THAN FORECAST.  THIS PRECIPITATION MAY ALSO LEAD TO A STRENGTHEN OF
THE OVERALL CAD.  IF THIS IS THE CASE...MORE ICE COULD ACCUMULATE
THAN FORECAST...ESPECIALLY ACROSS NW GA AND METRO AREA.
2. OVERALL MOISTURE WITH THIS SYSTEM IS NOT TOO IMPRESSIVE.  QPF
VALUES SUGGESTS AN AREA OF SIGNIFICANT ICE ACROSS THE NE GA MOUNTAINS.
HOWEVER...MOST OF THE MOISTURE THAT IS ADVECTING INTO THE AREA
AT THE 295K ISENTROPIC SURFACE HAS A RATHER LONG FETCH ACROSS THE
COASTAL SOUTHERN UNITED STATES.  WHILE ITS SOURCE IS FROM THE
GULF...THE FLOW COMES FROM THE EAST IN TEXAS.  THIS MAY...LEAD TO
LESS MOISTURE MAKING IT INTO THE AREA THAN FORECAST.  IF THIS IS THE
CASE THEN THE CURRENT ICE ACCUMULATION VALUES AND TOTAL QPF WOULD BE
OVERDONE.

SO THE FORECAST IS A COMPROMISE BETWEEN THESE TWO FACTORS.  GUIDANCE
OVERALL SUPPORTS THIS IDEA WITH SLIGHTLY LESS AMOUNTS OVERALL.  I DO
FEEL THAT THE WEDGE WILL HOLD ON SLIGHTLY LONGER THAN GUIDANCE
SUGGESTS...MOSTLY BECAUSE THE MICROPHYSICS IN MODELS ERODE THE WEDGE
FASTER THAN WHAT IS TYPICAL. THAT IS WHY I INCREASED THE TOTAL QPF
AND ICE AMOUNTS.  THIS SHOULD NOT BE MISCONSTRUED THAT THE FORECAST
REPRESENTS COMPLETELY WHAT COULD HAPPEN AS DESCRIBED IN POINT 1.
LATER FORECASTS SHOULD HELP WITH THIS UNCERTAINTY. AS FAR AS
TEMPERATURES TOOK AN OVERALL BLEND OF ALL MODELS.

 

I marked out what i didn't want, lmao     forecasting  like glenn burns!

[eyewall]

At 24 hours:

 

[calculus1]

At 24 hours:

 

Well, that's definitely "juicier" than prior runs of the GFS.  Trending...

[packbacker]

At 24 hours:

 

 

 

Quite an improvement, this was the 6z run as a reminder...

 

[griteater]

Wow, that's a shame. I know you've got the whole crumudgeon schtick going,but I always count on you for some intellectual heft. Not this time. I don't know what your point is here, but here is mine: Last week, I was under a winter storm watch for 2-4 inches of snow and got nada. The reason: even when the NAM and Canadian said the driving rain at my house would changeover to snow, it remained just driving rain. In retrospect, the RAP and NAM gave clues -- they both showed a trend of warmer BL within 24 hours of the event. Sure enough, even when precip was heavy, I did not change over. Now, I'm seeing a similar trend -- late -- that cuts across what has been conventional wisdom in my area (which is different than yours). That conventional wisdom -- the CAD will lock in and keep the I-85/US74 crowd below freezing at the surface throughout. I'm suggesting that the conventional wisdom is wrong is something to consider.

bean, I really like GFS MOS for surface temps.  18z for Shelby has temp of 32 and dewpoint of 13 at 1PM, and temp of 33 and dewpoint of 19 at 4PM.  I think this cold air mass is the real deal.  While surface winds are out of the SE, they are very light...in the 3-5mph range.  Having said all of that, I would agree that it is always, always prudent to question temperatures.  

 

http://www.nws.noaa.gov/mdl/forecast/text/state/NC.AVN.htm

 

Now precip, that is a whole different matter....I'm going with zilch to very light for our area.

[Shawn]

Looking at the GFS the wedge goes totally poof around here really fast.  It's a model known to fade CAD too quickly.

[beanskip]

Once again the 10m surface line trends north. There is now no place in SC north of that line once the main precip moves in. Just this morning, the 6z GFS 10m line dipped into SC at 21z.

[beanskip]

Thanks -- I know that a saturated column means temperature and dewpoint eventually meet and kiss -- guess the precip isn't heavy enough for that to happen this time?

Anyway, point well taken -- you clearly understand the point I was making. Nice to see a bit more juice in the 0z GFS.

bean, I really like GFS MOS for surface temps.  18z for Shelby has temp of 32 and dewpoint of 13 at 1PM, and temp of 33 and dewpoint of 19 at 4PM.  I think this cold air mass is the real deal.  While surface winds are out of the SE, they are very light...in the 3-5mph range.  Having said all of that, I would agree that it is always, always prudent to question temperatures.  

 

Now precip, that is a whole different matter....I'm going with zilch to very light for our area.

[Shawn]

Once again the 10m surface line trends north. There is now no place in SC north of that line once the main precip moves in. Just this morning, the 6z GFS 10m line dipped into SC at 21z.

 

I can't find the links but generally after updates I believe the GFS underdoes CAD type events/temps and erodes them too quickly and the NAM overdoes them.  You'll probably be okay.

[beanskip]

And to wit, 0z GFS sounding still clearly freezing rain, despite closenesso of 10m freezing line to my driveway.

bean, I really like GFS MOS for surface temps.  18z for Shelby has temp of 32 and dewpoint of 13 at 1PM, and temp of 33 and dewpoint of 19 at 4PM.  I think this cold air mass is the real deal.  While surface winds are out of the SE, they are very light...in the 3-5mph range.  Having said all of that, I would agree that it is always, always prudent to question temperatures.  

 

http://www.nws.noaa.gov/mdl/forecast/text/state/NC.AVN.htm

 

Now precip, that is a whole different matter....I'm going with zilch to very light for our area.

[beanskip]

It's not that I even care that much, it's just that I felt like there was a bit of a herd mentality last week buying into this notion that as long as you have a strong upper low, temps will eventually crash. This is almost the exact opposite situation, but I don't want it to be just assumed that because we have a nice CAD that all precip will be frozen.

I can't find the links but generally after updates I believe the GFS underdoes CAD type events/temps and erodes them too quickly and the NAM overdoes them.  You'll probably be okay.

[Shawn]

Charlotte, NC when light precip has beforehand begun per 0z GFS:

 

[Jon]

This is going to be a now cast event, doesn't matter what the GFS says at this point or how it trends. Once this moisture starts to pop off over Oklahoma and into Arkansas the short range models such as the HRRR will be more telling how much moisture is going to fall on the eastern side of the apps and into NC. It's just going to throw out solutions until we get some decent OBS...don't get your hopes up seeing this trend an extra tenth of an inch...it's likely not the case, IMO.

[superjames1992]

Could this be a situation where the GFS' lower resolution is causing it to miss some subtle features that the NAM and RGEM are picking up on?  The GFS just paints a uniform 0.1-0.25" across almost all of NC, which seems unrealistic.

[Shawn]

Could this be a situation where the GFS' lower resolution is causing it to miss some subtle features that the NAM and RGEM are picking up on?  The GFS just paints a uniform 0.1-0.25" across almost all of NC, which seems unrealistic.

 

As Jon just said the Hi-Res models are much more accurate over these long range deals.  I'd check out the RAP, HRRR, and Hi-Res4km (its on penn state ewall) from here on out.

 

EDIT: the sref still seems to be incorrect (but hey, every model is right sometimes)

[beanskip]

THIS is what I'm talking about. Last week, I'm watching the RAP/NAM trend farther north with the 5h low and I KNOW this means temp troubles for the i-85/US 74 folks and all I heard was nowcast nowcast yada yada. Yes, there is a danger in watching models when it's already sleeting outside. And the RAP, for example, had a terrible precip field, even an hour before the actual weather. BUT, there were trends that could have (should have) told us that a) the southern most areas of projected snow were in trouble because the 5h low shifted north and the models were increasingly pessmistic about BL issues. I'm not picking on you at all and you are mostly right, but these short-term model trends can still be important -- heck, they don't always have to be bad either!

This is going to be a now cast event, doesn't matter what the GFS says at this point or how it trends. Once this moisture starts to pop off over Oklahoma and into Arkansas the short range models such as the HRRR will be more telling how much moisture is going to fall on the eastern side of the apps and into NC. It's just going to throw out solutions until we get some decent OBS...don't get your hopes up seeing this trend an extra tenth of an inch...it's likely not the case, IMO.

[calculus1]

As Jon just said the Hi-Res models are much more accurate over these long range deals.  I'd check out the RAP, HRRR, and Hi-Res4km (its on penn state ewall) from here on out.

 

EDIT: the sref still seems to be incorrect (but hey, every model is right sometimes)

 

Yeah, you can check out the Hi-Res 00Z NAM simulated radar on Allan's site here:

 

http://raleighwx.americanwx.com/models/nam4km/00zNAM4kmreflectivity_SE.html

[NCSNOW]

For the record their isn't a model at the current time showing less than .24 for the immediate triad unless the ukie and euro paint something smaller in the next short while. Just reporting what's being thrown out and I lean moreso on short range as opposed to globals at this timeframe. Of course it all comes down to radar and overcoming the virga to see what actually ends up verifying. I for one wouldn't hedge bets on this being an overachiever precip wise. Shows how desperate times have become when we're sitting here chasing a possible freezing drizzle event.

[packbacker]

This is going to be a now cast event, doesn't matter what the GFS says at this point or how it trends. Once this moisture starts to pop off over Oklahoma and into Arkansas the short range models such as the HRRR will be more telling how much moisture is going to fall on the eastern side of the apps and into NC. It's just going to throw out solutions until we get some decent OBS...don't get your hopes up seeing this trend an extra tenth of an inch...it's likely not the case, IMO.

 

I was in DC last night and the RAP did really good within 12 hours of the snow DC had last night.  When we wake up tomorrow the RAP should be dialed in.  I like the RAP better than the HRRR but it will be interesting to see which one performs better tomorrow.

[griteater]

Could this be a situation where the GFS' lower resolution is causing it to miss some subtle features that the NAM and RGEM are picking up on?  The GFS just paints a uniform 0.1-0.25" across almost all of NC, which seems unrealistic.

Yeah, it is unrealistic, but no matter how you slice it, it says that the ceiling for this event is low (for this area anyway)

[MotoWeatherman]

Big time bust potential here in NGA.  Both the GFS and NAM came in a lot warmer at the surface and drier.  Neither model keeps the sub freezing sfc until 21z anymore while both had shown this consistently.  Combine that with the QPF concerns means we could have a non event here and school has already been cancelled in many counties around MBY.

 

Is it possible we have two straight winter storm warnings and we don't see anything at all?  It's possible.  I still think about 1/10" of ice tomorrow up here in far N GA but I am concerned about that even happening.

 

But hey I still have the 0z RGEM to cling to if I we want anything of any significance.

[griteater]

Surface temps are blah,not goof for any cold storm.Big warm nose is poking in.

What does this mean?

[packbacker]

RGEM has been very consistent, here is the precip map, it wants to paint a nice swath (0.25-0.40") right through the piedmont/central NC

 

 

[SN_Lover]

Is is me, or are the models trending towards the GGEM....

[SN_Lover]

RGEM has been very consistent, here is the precip map, it wants to paint a nice swath (0.25-0.40") right through the piedmont/central NC

it's been darn consistent for DAYS now 

[Jon]

THIS is what I'm talking about. Last week, I'm watching the RAP/NAM trend farther north with the 5h low and I KNOW this means temp troubles for the i-85/US 74 folks and all I heard was nowcast nowcast yada yada. Yes, there is a danger in watching models when it's already sleeting outside. And the RAP, for example, had a terrible precip field, even an hour before the actual weather. BUT, there were trends that could have (should have) told us that a) the southern most areas of projected snow were in trouble because the 5h low shifted north and the models were increasingly pessmistic about BL issues. I'm not picking on you at all and you are mostly right, but these short-term model trends can still be important -- heck, they don't always have to be bad either!

While I agree with your particular case, I think the "models you should look at" vary depending on the type of storm and what the models tend to do with those types. For instance, with an ULL like last week I would indeed look at the GFS and NAM the night before the event, for the very same reasons you're saying that you were looking at the 5h. For this particular event I think it's best to look at the short range models because of how much energy this system is lacking and because it's a "smaller" system that mind you hasn't even formed yet. I'm not saying I know anything about which models handle ULL's better and which don't, because I don't know, but I have a hunch that trends in the NAM and GFS would still be important with a system like that.

 

I was in DC last night and the RAP did really good within 12 hours of the snow DC had last night.  When we wake up tomorrow the RAP should be dialed in.  I like the RAP better than the HRRR but it will be interesting to see which one performs better tomorrow.

I heard about the RAP in DC, possibly from you or on twitter, can't remember...some people discount short range models but it pretty much won for DCA.