Snow Storms...They/re Only A Week Away

5H Z and MSLP


85H T and MSLP

MR models are at it again with teaser progs for this winter/s first significant EC snow storm and as usual...it/s just a week away.

This 'now-you-see-it...now-you don/t' scenario has played itself out several times this season. Sooner or later...odds are the atmosphere will get around to pulling the trigger instead of our thumb.

Today/s D+7 prog from the ECMWF depicts a classic cold air damming signature over the NE with strong arctic HIGH pressure centered over the Upper Great Lakes...supported downstream by an area of mid-level confluence...multiple sea-level isobars nosing SW along the Appalachian Mtns as far south as the GA / SC border...and an inverted trof along the SE seaboard indicative of a coastal front.



One big problem with the setup @ this point is the progressive nature of the flow regime as multiple short-waves are shown embedded in the westerlies. This suggests the storm might not be inclined to hang a left and hug the coast as it continues NE.

Snow Moon

Native American folklore holds February/s full moon...which occurs tonight...was known as the Full Snow Moon...supposedly b/c the heaviest snowfalls occurred during the second month of the year.

Maybe the tribes of northern and eastern America called it that...maybe they didn/t. All too often...Anglo-saxons have come up with pleasing stories about native peoples with little or no basis in fact.

Nevertheless...it would make for better folklore...regardless the origin...if it were based on observations that February is the climatologically favored time of year for good snows in the NE.

Too bad it/s not so. For most stations in the NE CONUS...January has the highest period of record normal (PORN) snowfall. In some cases...the difference between one month and the other is small...and maybe a statistical test on the difference between the two means would fail to reject the null hypothesis...but the data suggest the Snow Moon probably belongs to January.

Forecast Credibility

Wx web logs...forums...and message boards were all aflame yesterday with full-throated forecasts for a 'significant' snowfall in advance of the winter storm coming out of the GOM this evening.

Amateurs and pros alike hoisted 48-60 hour forecasts for wide swaths of 4-8" (meets and exceeds NWS warning criteria for heavy snow)...pockets of 6-12"...and 8+".

In light of more recent information...these collectively absurd forecasts have little chance of verifying.

Back in the day...forecasters knew better than to post snowfall forecasts with such long lead times. And they knew better not to segue the MR with the SR. Details portrayed in the the MR are pure fantasy. Timing...intensity...centers of circulation depicted in the MR are purely guidance of impending events. Same is often true about the SR progs...but they are much closer to the truth.

The lead time for a snowfall forecast with the best chance of verifying should be issued no more than 24 hours before the event. The current state of NWP is such that timing...intensity...and centers of circulation is too crude to attempt a forecast at such an extended time range.

A forecaster/s credibility needs to be preserved if they are to be taken seriously. Issuing a snowfall forecast more than two days before the first flakes fly is frought with danger and should be always be avoided.

FWIW - Verification of NCEP Week2 5H Z Anoms

Remember this?


It/s NCEP/s Ensemble Mean forecast for 500 mb height anomalies VT 1/26/2007...issued January 12. So...how good were the action center predictions?

500 mb height anomalies

Negative height anomalies...
...over NE CONUS verified farther N and E over the CN Maritimes
...over SW CONUS had minor displacement error N and W
...along and W of the IDL verified along IDL but farther E over the Aluetians
...over Greenland missed the NAO ridge.

Positive height anomalies...
...NW CN verified spot on
...ATL along ~40°N missed broad trof from CN Maritimes to NW Africa.

Good skill with PNA-type ridge-W...but completely missed the NAO dipole.

500 mb heights

Coastal Teaser #4

Click to animate. Loop speed = 1 frame / 3 sec

Wave forms on tail end of strong baroclinic zone courtesy arctic frontal boundary across the GOM. Dynamics from hot STJ provides the UVM.

Forecast problem is whether the storm will chug harmlessly out to sea or be held closer to the coast from the fx of a shortening wavelength.

Trying to forecast the timing and intensity of the numerous short-waves rotating around the PV is like playing roulette...even at shorter time ranges.

Sooner or later...something good has to come from the combination of the ern 2/3 of the CONUS flooded and continualy refreshed with arctic air and a hi-amp...short wavelength pattern.

Japan Also Waits for -AO

Japan is on the verge of breaking a long-standing record for latest snowfall.

The current record was set in 1876...when winter/s first snowfall was observed on February 10.

Like much of the CONUS...Japan has experienced a relatively mild winter. The anomalous warmth has been attributed to the persistently positive phase of the AO and ENSO.

Cold air is contained @ hi-latitude during +AO. +ENSO keeps HIGH pressure anchored off Japan/s SE coast in a fashion similar to a La Nina ridge over the SE CONUS.

Resurgent SOI

(Updated below)

Latest 30-day moving average of the SOI is -9.7.

The last time the SOI was that low was the 30-day period ending November 15.

Update:
The -9.7 SOI value is in the top 40% of all negative values since the index went negative last May. It/s also in the top third since the +ENSO began, base on NOAA/s definition (>= 0.5°C), in late September.

FWIW - Week2 5H Z Anoms

Week2 500 mb height anomalies from the PSD, ECMEF, and NCEP (VT 2/9/06).


Good agreement wrt Ridge-W / Trof E over CONUS...but does it foreshadow any snow?

Some support for -AO tripole with above normal heights over the Pole and below normal heights INVOF Aluetians. Height anomalies in the ern ATL don't fit the pattern...so it/s two out of three.

CPC index forecast from OP GooFuS (blue bars) and the Ensembles (yellow band) suggest at AO will be least one standard deviation below the mean; however, the coefficient of determination (R²) for Op (26%) and Ensemble (21%) are meager. Less than 30% of the AO/s variability is forecast at D+14 by the model. The AO loading pattern can been seen here.



One of the
NAO/s dipoles finds above normal heights over Greenland @ D+14 ...although not a strong action center. Not much of a weakness across the ATL Ocean...either. CPC/s NAO forecast has the index near zero @ VT. The Op (Ensemble) D+14 forecast has an R² of 21% (7%). Numbers like those don/t lend themselves to awe inspiring confidence.

Note the current state of the NAO; strongly negative which goes along way in explaining why there has been so few EC snowstorms to date. The UA flow regime responsible for the intrusions of arctic air is too strong to support nearshore EC cyclogenesis. The winter storms are out there, it/s just that they/re too far out there...going through their explosive development well offshore.



The PNA dipole of above normal heights over SW CN and below normal heights INVOF the Aluetian Is. is well supported by the three models. PNA index is not particulariliy high -- less than one standard deviation above the mean -- but ridge-W is a 'typical' mid-winter feature. The D+14 forecast for slightly above normal PNA suggests a continuation of arctic airmasses invading the lower 48.



The OP GooFuS captures only 19% of the PNA/s variability; however...the Ensembles are much better (~60%) at getting the magnitude and direction correct.

The current D+14 forecasts has the AO negative to strongly negative..the NAO neutral...and the PNA weakly positive. A recent four-part study of past NEWxSFC storms suggested the best snowfalls...on average...occurred when the AO was positive or when the AO and PNA were both positive. The lowest snowfalls occurred when the AO was negative and the NAO positive.

The present forecast combination of -AO and +PNA @ D+14 has not produced the best Contest snowfalls. Instead of big snows in the E by the end of Week2...what may be more likely is a continuation of arctic cold and arctic dry.

Coastal Teaser #3

Most promising MR prog in weeks!

Today/s 12z ECMWF coughs up a long overdue Miller A storm for the EC that has a lot going for it. Even though the chart is not 'perfect,' that/s a good thing at this time range. The last thing you want is perfection in the MR b/c the VT analysis won/t look anything like the 216 HR chart.

Nonetheless...there/s little doubt about the antecedent environment across the ern CONUS that will feature plenty-o-arctic air and a highly amped UA flow regime with short wavelength.

Locked & Loaded

A four Rossby-wave configuration is stationary...meaning once established...it/s not going anywhere...anytime soon. Arctic cold in the E has arrived and the ECMWF/s D+10 prog suggests it/s inclined to take its shoes off and sit a'spell.

In the run-up to D+10...storms are advertised to continue their explosions over the open waters of the wrn ATL due in large part to the rapid succession of upstream kickers. The interval btwn these nrn stream short waves is too short to allow any one of them to dig in its heels over land; however...


...the ECMWF/s D+7 prog shows an important break in the wave train interval that would allow a different scenario to evolve over the EC during the first week of February...just in time for the full Snow Moon.

As the short wave climbs the AK ridge...the ridge amplifies. A second short wave has already rounded the crest and is poised to descend into the LW trof on a direct-deposit trajectory toward the wrn GOM. Unlike previous short waves that are being abruptly kicked downstream and subsequently well offshore into the wrn ATL...this one has ample time and opportunity to dig deep into the base of the full-latitude trof b/c the upstream short wave is still ascending.

The increasing amplitude of the AK ridge has the downstream effect of shortening the wavelength over CONUS...which would allow cyclogenesis to occur much closer to the EC than earlier cyclone deepenings.