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Area forecast discussion...corrected 
National Weather Service Buffalo New York 
412 am EST Friday Nov 21 2008 


Synopsis... 
a northwesterly flow of very cold air will continue across western 
and central New York through at least Saturday. This will result in 
temperatures remaining well below normal right through the first 
half of the weekend...and narrow bands of heavy lake effect snow 
southeast of lakes Erie and Ontario. 


&& 


Near term /through tonight/... 
radar shows lake snows continuing in northwest flow convergence off 
Lake Erie with visibility occasionally below 1/2 mile on the thruway 
at Dunkirk. The NAM shows it continuing through the morning then 
weakening in the afternoon...then an intense northwest-southeast 
band drifts east across the area tonight. Thus we are continuing the 
lake snow warning. 


Off Lake Ontario the WRF and NAM show good low level Omega and lake 
snows organizing this morning. With the Arctic air and winds lining 
up at 290 to 300 degrees, conditions are favorable for intense lake 
snows by afternoon. We have upgraded the watch to a lake effect snow 
warning with up to 9 inches in across northern Cayuga County and 
nearby Wayne and Oswego counties. Winds veer to 320 this evening, 
but both the NAM and WRF show a northwest-southeast band across Oswego County... 
and unusual positioning, but the GFS has it across the same area as 
today, so we have the warning continuing into Saturday with another 
5 to 9 inches tonight. 


Elsewhere, the snow showers will be likely in the Rochester area in 
the northwest flow today and a better chance for accumulation 
tonight as the winds aloft veer to 310-320. Buffalo will see only 
scattered light snow showers with some showers likely this afternoon 
drifting south from Lake Ontario. 


&& 


Short term /Saturday through Sunday/... 
as is the case for the near term period...lake effect snows will 
also remain the main forecast concern in the short term portion of 
the forecast. 


In terms of the overall synoptic pattern...the deep upper trough 
responsible for the late week chill will track from western 
New England Saturday morning to the Canadian Maritimes by Saturday 
evening...before further lifting out across eastern Quebec province 
Saturday night and Sunday. As this occurs...low-level ridging over 
the western Great Lakes Saturday morning will gradually slide east 
to southern Ontario by Saturday evening...before flattening out and 
drifting to the East Coast during the course of Saturday night and 
Sunday. 


As a consequence of this evolution in the pattern...a northwesterly 
flow of very cold air /850 mb temperatures of -14 to -16c/ during Saturday 
will gradually turn more westerly Saturday night...then more 
southwesterly on Sunday while 850 mb temperatures warm by about 4 degrees c 
each 12 hour period. This warming aloft...in conjunction with 
increasing drying/subsidence/increasing shear associated with the 
advancing low level ridge will result in the synoptic pattern 
becoming less favorable for heavy lake effect snows with time... 
particularly from later Saturday afternoon on through Sunday. For 
this reason...the lake effect snow warnings currently in effect in 
the near term period will only extend through Saturday at the 
present time. 


Off Lake Erie...expect heavy lake effect snows to continue to target 
Chautauqua and Cattaraugus counties during Saturday morning...before 
gradually weakening and drifting/expanding northward into southern 
Erie and Wyoming counties Saturday afternoon and Saturday night in 
response to the aforementioned drying/subsidence and backing 
steering flow. Have maintained categorical to likely probability of precipitation through 
Saturday within the expected lake effect areas...before backing off 
to chance Saturday night as the large-scale environment becomes 
increasingly unfavorable. That said...still expect some scattered 
snow showers to persist into Sunday morning across areas south of 
Buffalo and Batavia...before dissipating completely by late morning 
due to continued drying/rapidly increasing shear. 


Off of Lake Ontario...expect the same general evolution in the lake 
snows to occur...albeit about 6 to 12 hours after that for Lake 
Erie. Persistent lake snows should continue to focus on the 
Wayne/northern Cayuga/Oswego corridor into Saturday evening...before 
gradually weakening and drifting northward later Saturday night and 
Sunday. Given that the large-scale environment will remain more 
favorable here...have maintained categorical probability of precipitation through Saturday 
and likely probability of precipitation through Saturday night and into Sunday 
morning...before dropping to chance as the activity breaks up into 
scattered snow showers during late Sunday morning and Sunday 
afternoon. In contrast to the flow over Lake Erie...the flow across 
Lake Ontario will be somewhat less sheared on Sunday...and thus 
expect that some of the remnant activity should survive the trip north 
into the Watertown area Sunday. 


&& 


Long term /Sunday night through Thursday/... 
by Sunday evening...this weekend's upper low will finally be in the 
process of lifting out across Labrador. In its wake...a low 
amplitude upper ridge will be building across the lower Great Lakes 
region...in advance of yet another upper low digging southward into 
the upper Mississippi Valley. As a result...a warm air advection 
pattern will continue across the area at all levels...and will 
probably be strong enough to touch off some snow showers from Lake 
Ontario and the north country northward. Further south...the 
remainder of the area should generally remain dry. 


As the aforementioned upper low is expected to dig further 
southeastward into the Great Lakes region and close off during the 
course of Monday...with its associated surface reflection reaching 
southern Ontario by late in the day...before drifting east across 
New York state Monday night. The strong warm air advection pattern 
associated with the surface and upper lows warrants likely probability of precipitation for 
both the Monday and Monday night periods at this time...with 
boundary layer temperatures warming enough to support a rain/snow 
mix on Monday...then cooling enough Monday night to allow for the 
precipitation to change back over to mainly snow. 


As was the case yesterday afternoon...the latest runs of the GFS and 
European model (ecmwf) both show the closed upper low remaining across our region on 
Tuesday...before gradually lifting away from the area during the 
remainder of the week...though the GFS is considerably slower to 
lift the low out than the European model (ecmwf) and other medium range guidance. 
Given the close proximity of the system...will maintain high chance 
probability of precipitation Tuesday and Tuesday night...then slowly lower probability of precipitation back through 
the chance range Wednesday into Thursday as the low slowly lifts 
away from the area. Temperatures will remain below normal though the 
period...but should warm enough each day to promote a rain shower/ 
snow shower mix during the late morning and afternoon hours...with 
snow showers remaining the predominant ptype the remainder of the 
time. 


&& 


Aviation /09z Friday through Tuesday/... 
look for boundary MVFR/VFR ceilings and visibilities for the most part at 
all of our taf sites through the 24 hour taf period with off and 
on snow showers briefly restricting visibilities further. The best 
likelihood of lower conditions at Jamestown and Rochester mainly 
this morning and tonight...with the least chance at Watertown. 


Outlook... 
Saturday...local IFR in lake effect snow southeast of the lakes. 
Sunday...VFR. 
Monday...MVFR/IFR in rain or wet snow. 
Tuesday...MVFR in snow showers. 


&& 


Marine... 
a northwesterly flow of very cold air will continue across the lower 
Great Lakes region right through Saturday. At this point...it 
appears that the flow will become just strong enough to result in 
wave heights reaching small craft criteria along the southeastern 
shore of Lake Ontario later this afternoon and this evening...and along 
the southeastern shore of Lake Erie later tonight and Saturday. For 
this reason...small craft advisories have been hoisted for the 
corresponding nearshore marine zones. 


&& 


Buf watches/warnings/advisories... 
New York...Lake effect snow warning until 6 PM EST Saturday for 
nyz004>006. 
Lake effect snow warning until 6 PM EST Saturday for nyz019- 
020. 
Marine...Small Craft Advisory from 1 am to 12 PM EST Saturday for 
lez040-041. 
Small Craft Advisory from 2 PM this afternoon to 11 
PM EST this evening for loz043-044. 


&& 


$$ 


Synopsis...jjr 
near term...apb 
short term...jjr 
long term...jjr/Sage 
aviation...Hitchcock/sfm 
marine...jjr 

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