First call on Ohio Valley storm

[PurdueWx80]

Still prone to shifts in track given reasons listed below....here goes:

The GFS has come more in line with the Eta this evening, with a weaker surface low and lower precip totals (that are generally located further south and east). The MM5 is further east and weaker than prior runs, although it still brings precipitation further north than both the Eta and GFS. Of note, the Eta now fails to close off a circulation at 850 or 700 mb, meaning the chances of extreme heavy snow would be less, even though it's QPF associated with warm air advection and convergence along the 850 front is still quite high. No closed off circulation at these lower levels, and a weaker system at 500 mb lessens the chance of heavy snow in the deformation zone (in fact it doesn't even seem to have precip associated with one). For what it's worth, the high-resolution extended RUC models have a further NW track, bringing heavy snow from north-central AR to southern St. Louis to central and northeastern IN with far southeastern IN and southern OH mostly seeing rain. Temps across the front go from the 10's in western KY to the 50's just 50 miles to the east!!! That's one hell of a front! These RUC models are experimental and aren't always the best, plus their domain is much smaller so once a feature goes into it, the new data may change the outcome more than necessary. If the next 2 or 3 runs show continuity, I may give them credence since their resolution is so much higher (not always a good thing, but generally works out). The final outcome, as you will see below, will come down to the vort max/upper low just now coming on shore in Alaska and British Columbia - there is PLENTY of time for all models to change, and the differences seen now are reflected in how each one handles this energy (which may not have been well sampled by 00Z data). Regardless, warm air advection and upper divergence (from difluent stream, coupled upper jets and especially strong anti-cyclonically curved northern stream jet) alone argue for heavy totals somewhere in the Ohio Valley. However, moisture return has not been nearly as vigorous as was forecast this morning along the Gulf. Instead of low to mid-60's dewpoints, they are only in the low to mid 50's with a few stations approaching 60 now. I think this may be responsible for the slightly smaller QPF values shown as the models are realizing that cold, dry air above the warm Gulf isn't equilibrating as quickly as once expected. The GFS also isn't suffering from as much feedback.

As of now, I think the precipitation will come in two waves, via the GFS and MM5 solutions - round one would consist of precip associated with warm air advection (VERY strong 850 flow), moisture advection, convergence and strong frontogenesis that would begin early Wednesday morning. This will occur when temperatures are more critical (closer to or slightly above freezing in southern areas) but the amount of vertical lifting would more than likely cool the column to or below freezing down to the Ohio river, changing brief rain/frozen precip over to plain snow. Brutally colder air will also begin to infiltrate the region by Wed. morning behind the secondary arctic front (front with air of Pacific origins is what system will originally form on...it is associated w/ the snow in WI and MI now). Snow rates in this area could approach 2"+/hour for a couple of hours Wed. in southern areas of snow, with 0.5-1.5"hour rates further north. Thundersnow is not out of the question as very warm air is closeby - totals from the WAA regime would approach 2-6+". Round two would begin later Wednesday and persist through the night - it would consist of a deformation zone assuming a stronger upper vort max induces closed circulations from 850-700 mb. Remember, much colder air will have been ushered in by this point, increasing the snow:liquid ratios. Snow rates may approach 1-2" per hour in the core of this zone as well, with thundersnow again possible with the stronger scenario (of closed-off circulations). This will put down another 1-5" giving a swath of final snow totals that will range from 4" to 10" with pockets of 10-15" where the two rounds intersect (highest likelihood of this will run parallel to the Ohio River and will be approximately 20-50 miles north of it). Southern fringes will mix with sleet and freezing rain, which is clearly no better with expected liquid equivalent QPFs of 1.5-2". These areas will also have 1-2" of snow on top of the ice - best chance of this just south of the Ohio river (w/i approximately 20 miles), with all rain south of that. Snow will also fall in the mountains of northern AR and far southern/southeastern MO. My current guesstimate of the snow accumulation area will be bounded by a line running from near Tulsa, OK to Paducah, KY to Cincy to Cleveland to Detroit to Ft. Wayne and back to Tulsa. This is the general 2-6" area with the axis of 6-10"+ running from Carbondale, IL to Wilmington/Dayton, OH and up to Cleveland. Totals over a foot would be most likely further east on this axis (south-central IN to western Cleveland) once the dynamics have become more favorable for heavy snowfall.

This case will be an excellent example of when not to follow the 5400 meter thickness line for rain/snow - instead the 1000-850 and 850-700 critical thicknesses will be substantially farther south of the 1000-500 crit. thick. because of strong low-level cold air advection. Also, dynamic cooling of the column due to very strong vertical motions may act to produce very heavy snow rates in the strongest cores of precipitation (which may be south of the 1000-500 mb crit-thick as well.

[PurdueWx80]

Going to bed (way too late)...06Z Eta just now in and showing a stronger solution w/ a closed low at 850 mb and much better deformation zone in back of system (a la 1-2 punch seen in 00Z GFS and MM5). QPF values are also back up to 2" along the Ohio river...surface low mostly uneffected by these changes though. Will have more confidence in forecast tomorrow once 12Z data over NW US and W Canada is ingested into the models.

[PurdueWx80]

12Z Eta comin in similar to 06z with slightly stronger solution, but no real change in the layout of precipitation. Tomorrow will be very interesting on the southern fringes of the snow, because 1.5-2" of liquid is forecast. If dynamical cooling can come into play, these areas will see a heavy wet snow, w/ snow-to-liquid ratios only around 8:1, meaning 12-18" are possible. Of note is the fact that 00 and 06Z MOS guidance is now running 5-10 degrees warmer than actual temperatures in the Plains behind the front. This morning's Eta may have caught on, as it's forecast 1000-850 and 1000-700 mb critical thicknesses are now colder and slightly further south through the event. If this cooler-than-forecast air makes it down to the inverted trough/front in time, it will only increase the prospects of snow or frozen precipitation in the locales w/ high QPF along the immediate vicinity of the Ohio River. The 12Z run also shows the 2-round scenario w/ much of the precip falling in the overruning scenario, followed by a 1-4" band associated with deformation. May post one more time before I hit the road on the way to Louisville.

[PurdueWx80]

12Z GFS quite similar to the Eta, but it's precip shield still extends further north. Dry air will come into play on the northern fringes of the system, so the cutoff between snow and no snow will be quite stark. I still pretty much like my first ideas, and still think very isolated spots will see over a foot of snow (up to 18") in parts of IN and OH. Snow will extend from Tulsa back to the panhandle of TX as well, but generally should remain in 1-3" range there.

[Guest]

12Z GFS quite similar to the Eta, but it's precip shield still extends further north. Dry air will come into play on the northern fringes of the system, so the cutoff between snow and no snow will be quite stark. I still pretty much like my first ideas, and still think very isolated spots will see over a foot of snow (up to 18") in parts of IN and OH. Snow will extend from Tulsa back to the panhandle of TX as well, but generally should remain in 1-3" range there.

GFS = Stronger storm & Like the 06z further west from the 00z.

Tell me i dont have to post all the maps.

[PurdueWx80]

18Z Eta now quite interesting and has gone towards some mix of it's older runs and yesterday's GFS maps. The low ends up considerably further west in the end and is quite a bit stronger than the 12Z run (1004 vs 1000 mb now at 18Z Thursday). It has also closed off much stronger circulations up through 700 mb and the vort at 500 is more concentrated w/ the same coupled jet structure - looks pretty good to me right now. If 00Z data doesn't affect the models too much than this will be a snow for the Ohio Valley to remember for years. I'm still concerned of some surprise totals close to 20" with this storm, much like in Jan 17 1994 in these regions.

Back home at the parent's for the week w/ a SLOW connection...I-65 was very busy, and the crews were already out spraying brine on the road even as it is in the 50's down here. It is almost pointless down here because the precip will be rain for a few hours, acting to wash it all off before the snow starts. Be careful out there tomorrow!

[beachbummer]

Sheesh.......trying to get son to decide on flight out of L'ville, KY, Wed or Thurs.....any insight?

[PurdueWx80]

:grr: Sheesh.......trying to get son to decide on flight out of L'ville, KY, Wed or Thurs.....any insight?

It may be quite icy at the airport in Louisville tomorrow (Wed), then snow will get dumped on the ice tomorrow afternoon and night. This could potentially close the airport for a bit, but his best chances would either be early Wednesday morning, or late Thursday afternoon/evening.

[beachbummer]

Thanks....I really appreciate the info!....Makin for one less stressed mamma!

[isobar]

Thanks for the model discussion, Purdue. A very good read.

The daily snow record for Evansville is 9". Hoping to get close to that, along with a couple good thundersnow rumbles.

[breeze]

Thanks, Purdue! I see, this evening, that the GFS and
ETA are both consistent with precip in my area. But, I
couldn't get the MM5 to load on the site that I was
viewing! Where did you get your MM5, if you don't mind
my asking...?

[PurdueWx80]

Thanks, Purdue! I see, this evening, that the GFS and
ETA are both consistent with precip in my area. But, I
couldn't get the MM5 to load on the site that I was
viewing! Where did you get your MM5, if you don't mind
my asking...?

I saw it from Accuweather's pay-site. However, check out http://www.wxcaster.com/models_main.htm - there are NUMEROUS model runs there with all sorts of excellent products (almost too many to look at). The MM5 there only goes out 48 hours, but all 4 daily runs are available. We're in the period of time now where 48 is all we need anyway!

[breeze]

Thanks, Purdue! 48 hour out, especially in my
hard-to-forcast area, is all I trust, anyway!

[Guest]

Sorry about the earlier post Perdue. But as i am certain you have seen the eta came in at 18z further west more so with the precip and the gfs has now held its ground for 3 runs going back to 06z this morning. Which is further to the nw then the 00z run last night.
I'll wait though to see the 00z tonight before jumping the gun on anything BUT if the 00z holds onto the same ideas that the 18z showed us my fear is that those along and just to the north of the OH River may see a longer period of Freezing rain. And the same could be said from Cincy up to Columbus and even Cleveland. FWIW (Going off the unisys site) the EC came in further to the nw as well taking the storm now up into Central OH.


*EDIT* just noticed the discussion with wxguy25.

Great discussion guys.

[beachbummer]

INZ076>079-083-084-089-221000-
CRAWFORD-DUBOIS-JEFFERSON-ORANGE-PERRY-SCOTT-WASHINGTON-
INCLUDING THE CITIES OF...JASPER
10 PM EST TUE DEC 21 2004

...WINTER STORM WARNING IN EFFECT FROM 3 AM WEDNESDAY TO 6 AM EST
THURSDAY...

THE NATIONAL WEATHER SERVICE IN LOUISVILLE KY HAS ISSUED A WINTER
STORM WARNING. LIGHT RAIN WILL QUICKLY OVERSPREAD THE ENTIRE AREA.
THE RAIN IS FORECAST TO CHANGE TO ALL SNOW BEFORE SUNRISE. THE SNOW
MAY BECOME HEAVY AT TIMES WITH ACCUMULATIONS OF 6 TO 10 INCHES
POSSIBLE. AREAS THAT WILL BE IMPACTED WITH THIS ACCUMULATION OF
SNOW INCLUDE THE CITIES OF JASPER...PAOLI...SCOTTSBURG AND MADISON.

TRAVEL WILL BECOME EXTREMELY HAZARDOUS LATE TONIGHT AND DURING THE
DAY ON WEDNESDAY. THOSE WITH HOLIDAY TRAVEL PLANS IN THE WARNED AREA
ARE ADVISED TO MONITOR THE LATEST FORECASTS AND ADJUST TRAVEL PLANS
AS NEEDED. IF THIS IS NOT POSSIBLE...USE EXTREME CAUTION IF TRAVEL
IS UNAVOIDABLE.

STAY TUNED TO NOAA ALL HAZARDS RADIO...COMMERCIAL MEDIA OUTLETS OR
YOUR CABLE TELEVISION PROVIDER FOR THE LATEST INFORMATION CONCERNING
THIS POTENTIALLY DEADLY WINTER WEATHER SITUATION.

THIS PRODUCT...ALONG WITH OTHER WEATHER...HYDROLOGICAL AND CLIMATE
INFORMATION...IS AVAILABLE ON THE WEB AT HTTP://WWW.CRH.NOAA.GOV/LMK
OR AT HTTP://WEATHER.GOV.

$$