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Jan 20-22 Threat Potential Part 2


am19psu

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What is causing this storm to shift further south? It is not like we have a big -NAO or -AO to force the storm track to the south. The PNA is positive and the NAO is near neutral. I don't get it.

Ant... I think part of the problem is what the depiction is in the Western Atlantic.. the flow is too much oriented from a west to east fashion and it is a swift flow at that... also, the shortwave impacting us is propogating from west to east because there is equal amounts of energy on both the front and back side of the trof. Without a good advection of vorticity into only the base of the trof at a time frame before it reaches us, we cannot force it to be negatively tilted.. as such, this has consequences with the heightfield out ahead of it and cannot force the system to go on a more northerly trajectory.

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not when Rainshadow posted verification scores, shows how overdone NAM qpf has been this winter.

The wet bias is more pronounced farther out in forecast time and also at amounts reaching/exceeding the .50" threshold.

The 00z and 12z nam runs are more of it coming closer to the sref mean and model consensus than that's it for this one, what's next?

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I do realize that the chance of reaching warning criteria snowfall is not the best at the moment (6") up north but 0.5" QPF with high ratios could still be realized up north. Any chance of watches for the far north later today?

The chance of higher amounts are for eastern and north eastern areas. LI and SNE have a better chance at more QPF then areas to the west and north.

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The chance of higher amounts are for eastern and north eastern areas. LI and SNE have a better chance at more QPF then areas to the west and north.

I'm aware of where the best chance of higher QPF is as I'm more than capable of reading a model run. My question was pointed at rainshadow since I know he works for the NWS (Mt. Holly). Therefore the northereastern sections of the Mt. Holly CWA is my concern.

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I'm aware of where the best chance of higher QPF is as I'm more than capable of reading a model run. My question was pointed at rainshadow since I know he works for the NWS (Mt. Holly). Therefore the northereastern sections of the Mt. Holly CWA is my concern.

I'm not on the public forecast desk today, just took a quick look at bufkit soundings off the latest nam, the omega is OK in the snow growth area, but not the stuff 15-20:1 ratios are made of. I can't access a terminal to see the fgen banding potential with it, but just glancing at the other mass fields, I wouldn't think it would be that robust. But that's just the nam, have to see what the gfs and euro do as well.

Gotta get back to November, so if you have any other questions, please pm me, I'll try to answer them later.

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12z NAM text for NYC crew:

NYC: .30" - temps in the upper 20's

LGA: .30" - temps in the upper 20's

JFK: .29" - temps from 30-31.5

EWR: .33" - temps in the upper 20's

With those temps in the city area, ratios will not be that high. I would go with 9-11 to 1 ratios or 2.5"-3.5"

This is one of the widest misconceptions on this board. Surface temps DO NOT reflect snow ratios; temperatures in the dendrite snow growth regions do (around 600mb). In the NAM's case, ratios would be much higher than 9:1, and even 11:1. More likely 14:1. I haven't seen the NAM soundings but just looking at temps at 850mb, I can tell ratios would be rather high.

With that said, I believe the truth lies with an 00z GFS, Euro, and 6z NAM blend. Many are bashing the NAM for its most recent debacle in QPF output. What most are neglecting to acknowledge is the fact that the dynamics of this storm are much different. Every model has a difficult time forecasting lare scale cyclogenic events. These effects are more pronounced on mesoscale models because that's what they're made for. Overrunning events that do not blow up over our area are not the same at all. I'm sure the verification rates for the NAM's QPF output are much higher for overrunning events than for any cyclogenic, unstable event.

We are converging on a consensus. It seems as though the latest trends have been to hold pieces of the southern stream energy back before the northern stream arrives. Although I see no mechanism for this, it is plausible. The Euro may be overdoing this, as was suggested by BI earlier in the thread when commenting on latest water vapor imagery. I haven't seen the most recent imagery, but I would imagine that the Euro and its ensembles are still too far southwest. Most global models agree on a moderate event. In fact, all do except the Euro.

All this to say, don't discount the 12z NAM, or the 6z NAM for that matter. Both are very viable solutions and not spitting out ridiculous QPF results. This is easier for the models to handle. Let them do it.

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An excellent forecast.

Mt Holly AFD calling for 3-5" for Friday:

.SHORT TERM /TONIGHT THROUGH FRIDAY/...

-- Changed Discussion --HIGH PRESSURE WILL BUILD IN TONIGHT AND THURSDAY WITH MOSTLY FAIR

WEATHER AND SEASONABLE TEMPERATURES.

THE NEXT LOW PRESSURE SYSTEM WILL DEEPEN AS IT MOVES ACROSS THE

OHIO VALLEY AND MIDDLE ATLANTIC THURSDAY NIGHT AND FRIDAY. A

SHIELD OF SNOW WILL SPREAD OVER THE AREA WITH ACCUMULATIONS

EXPECTED. PRESENTLY 3 TO 5 INCHES OF SNOW LOOKS ON TARGET FOR MOST

AREAS. ACCUMS COULD BE LESS ACROSS THE DELMARVA...BUT IT IS STILL

TOO EARLY TO GET TOO FANCY WITH THE ACCUMS AND TRACK OF THE SYSTEM

YET.

THE DEEPENING LOW MOVES OFF TO THE NE FRI AFTERNOON AND A STRONG

WRLY FLOW OF COLD AIR WILL ARRIVE ACROSS THE AREA. THIS COLD AIR

WILL BE AROUND FOR THE WEEKEND.-- End Changed Discussion --

http://forecast.weat...1&highlight=off

Upton has similar wording, talking about advisory to perhaps warning level snowfalls...

.SHORT TERM /6 PM THIS EVENING THROUGH FRIDAY/...

-- Changed Discussion --SHEARED OUT TROUGH AXIS CROSSES EAST OF THE REGION TONIGHT...WITH

CANADIAN HIGH PRESSURE BUILDING TOWARDS THE REGION TONIGHT AND THEN

OVER THE REGION ON THURSDAY. GUSTY CAA WNW FLOW TONIGHT WITH

LOW-LEVEL DRYING AND CLEARING SKIES IN WAKE OF TROUGH AXIS. TEMPS

EXPECTED TO FALL INTO THE MID/UPPER TEENS ACROSS INTERIOR...AND

LOWER TO MID 20S COAST.

THEN FOCUS WILL BE ON PAC AND NORTHERN STREAM ENERGY OVER THE

CENTRAL PLAIN ON THURS...WITH MODELS/ENSEMBLES COMING INTO FAIRLY

GOOD AGREEMENT WITH TROUGHING DIGGING INTO THE DEEP SOUTH BY THURS

EVENING AND THEN NEGATIVELY TILTING UP THROUGH THE NORTHEAST ON

FRIDAY. SOME SUBTLE DIFFERENCES IN PHASING/TIMING/STRENGTH WHICH ARE

AFFECTING LOW PRESSURE EVOLUTION...BUT OVERALL IDEA IS THE SAME. AT

THE SURFACE...RESULTANT BROAD LOW PRESSURE DEVELOPING FROM THE GULF

TO THE TENNESSEE VALLEY LATE THURSDAY...LOOKS TO TRACK TO THE MID

ATLANTIC COAST BY FRIDAY MORNING...WITH AN INTENSIFYING COASTAL LOW

TRACKING NE TO THE VICINITY OF THE 40/70 BENCHMARK BY FRIDAY

AFTERNOON.

THE ABOVE MENTIONED UNCERTAINTY SHOWS ITSELF IN DIFFERENCES IN

EXACT DETAILS OF THE LOW AMONG MODEL GUIDANCE...PARTICULARLY WITH

TRACK AND INTENSITY. THE COMMON DENOMINATORS ARE THAT THIS WILL BE

AN ALL SNOW EVENT WITH A TRACK SE OF THE AREA AND WITH POLAR HIGH

LOCKED OVER CENTRAL US/SOUTHERN CANADIAN PLAINS INTO

ONTARIO/QUEBEC...AND IT IS A QUICK HITTING SYSTEM. THE POTENTIAL

FOR AN ACCUMULATING SNOWFALL IS HIGH...BUT AMOUNTS ARE STILL IN

QUESTION. AN ADVISORY LEVEL SNOW WOULD BE EXPECTED WITH A TRACK SE

OF THE 40/70 BENCHMARK AND LOW END WARNING SNOW WITH A TRACK

INSIDE. MODELS VARY AMONG THESE SOLUTIONS. CLIMATOLOGY ON THESE

QUICK MOVING SYSTEMS PUT THIS AT HIGH CONFIDENCE ON AN ADVISORY

LEVEL EVENT WITH A LOW END THREAT FOR WARNING LEVEL SNOWS. STILL

TOO EARLY FOR ANY WATCHES OR ADVISORIES...BUT MONITOR SUBSEQUENT

FORECASTS AS THE DETAILS SHOULD CLEAR UP. WILL ADDRESS IN HWO.

TEMPS DURING THIS PERIOD WILL RUN ABOUT 5 TO 7 DEGREES BELOW

SEASONABLE...GENERALLY HIGH IN THE MID 20S INTERIOR TO UPPER

20S/LOWER 30S CITY/COAST. LOWS IN THE MID/UPPER TEENS INTERIOR AND

LOWER/MID 20S CITY/COAST.-- End Changed Discussion --

http://forecast.weat...1&highlight=off

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Snow ratios probabilities based on NAM and GFS runs from 6z (this process is slow to update)

KRDG

NAM 9:1-15:1 33% / >15:1 62%

GFS 9:1-15:1 22% / >15:1 75%

KABE

NAM 9:1- 15:1 34% / > 15:1 60%

GFS 9:1-15:1 21% / > 15:1 77%

KPHL

NAM 9:1- 15:1 39% / > 15:1 53%

GFS 9:1-15:1 23% / > 15:1 73%

KTTN

NAM 9:1- 15:1 36% / > 15:1 57%

GFS 9:1- 15:1 23% / >15:1 73%

KLGA

NAM 9:1-15:1 37% / > 15:1 55%

GFS 9:1-15:1 25% / > 15:1 70%

KEWR

NAM 9:1-15:1 36% / > 15:1 58%

GFS 9:1-15:1 25% / > 15:1 71%

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Sounds like HPC thinks the NAM QPF was underdone?

12Z NAM EVALUATION

ANY INITIALIZATION ERRORS WITH THE NAM DO NOT APPEAR TO

SIGNIFICANTLY IMPACT ITS SOLUTION.

...CLIPPER DEVELOPING INTO NORTHEAST SHRTWV TROF WITH MID-ATLC NEW

ENG COASTAL SFC LOW ...

NAM HAS TRENDED FASTER AND SHARPER WITH THE DEVELOPING TROF

ESPECIALLY EARLY IN THE PD. THIS CORRESPONDS TO A FASTER/STRONGER

LOW DEVELOPMENT... BUT QPF TO NEW ENG DOES NOT SEEM TO BE

SIGNIFICANTLY DIFFERENT THAN 00Z CYCLE. THE LOW TRACK SEEN IN THE

NAM IS ON THE EAST SIDE OF THE SPREAD.

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