Scientific Forecaster Discussion
Area forecast discussion
National Weather Service northern Indiana
705 PM EST Monday Dec 22 2014
issued at 1127 am EST Monday Dec 22 2014
Two low pressure systems will move through the area this
week...bringing rain and snow to the region. Expect rain and
drizzle overnight tonight and Tuesday. On Wednesday...expect rain
to continue before changing over to snow late in the afternoon.
Snow is expected Wednesday night. Highs will be in the 40s.
Short term...(this evening through tuesday)
issued at 408 PM EST Monday Dec 22 2014
Area of rain associated with low level Theta-E advection/isent
lift/moisture convergence ahead of deepening Central Plains low
should continue to overspread our County Warning Area from west-east late this aftn/eve.
Another batch of widespread rain expected overnight as vorticity maximum
rotating around the upper low lifts NE through northern Illinois and occluding
surface front approaches from the west. Rainfall should be lifting NE
out of our County Warning Area Tuesday morning as dry slot wrapping around the low
spreads NE into our area. Low levels will likely remain saturated
with some weak lift continuing until occluded front passes... so expect
some light rain or drizzle will persist until midday...especially
across northern/eastern portions of County Warning Area. With warm front moving through the
area this period... temperatures should continue on a slow climb overnight and
Tuesday... reaching highs in the M-u40s Tuesday afternoon.
Long term...(tuesday night through monday)
issued at 408 PM EST Monday Dec 22 2014
..main focus of the long term is the storm system expected
Unfortunately new model runs continue to disagree on the secondary
surface low track for Christmas evening/Christmas day. The 12z GFS/Gem have
a stronger system moving from Kentucky at 12z Wednesday...to NE in/Northwest Ohio by 18z
Wednesday...and The Thumb region/Lake Huron by 00z Thursday. The 00z European model (ecmwf)/12z
NAM have a weaker surface low along the Gulf 12z Wednesday with a trough
across the Ohio Valley...then develop the surface low and lift it
into Kentucky by 18z Wednesday...with the European model (ecmwf) a bit stronger with the low. At
00z Thursday both models have the surface low situated over western
Ohio...with the NAM a bit further north. All of the models have a
closed/cut off low and negatively tilted trough in the middle
levels...though the NAM is the outlier keeping the cut off low over
southern Illinois versus the Gem/GFS/ECMWF have it over SW lower Michigan.
As mentioned in previous discussions...with the deepening low and
the negatively tilted trough want to lean towards the slightly
westward solutions of the Gem/GFS. Overall went with a Gem/GFS/ECMWF
compromise given these significant differences.
Tuesday night as the first system moves out cut probability of precipitation for the first
six hours as dry slot moves over the area. After 6z have increasing
trends again as secondary low approaches the area. Expect rain
through the late afternoon across a majority of the County Warning Area before the
change over to snow occurs...starting in the northwest near Lake Michigan.
Expect a majority of the snow accumulation to be in the 00z to 6z
time frame as temperatures fall and the deformation zone of the
deepening low swings overhead. Generally think the going forecast of
1 to 3 inches is still on track...mainly in the west and central County Warning Area
given mixed precipitation/warm ground concerns when the forcing is strongest.
Again though...uncertainty remains as to where the strongest forcing
will be. Taking a quick peek at the 12z European model (ecmwf)...it has come in line
closer to the Gem/GFS solutions...which may force the axis of
heavier precipitation further to the northwest. Those traveling on
Christmas evening should continue to monitor the latest forecasts.
Aviation...(for the 00z tafs through 00z Tuesday evening)
issued at 659 PM EST Monday Dec 22 2014
Large area of rain along nose of strong isentropic lift/ll moisture surge
was overspreading County Warning Area this evening west/southwestward expansion noted in
regional composite imagery. MVFR conds to start will deteriorate
going forward overnight west/at least IFR conds realized after midnight and
further reduction to LIFR likely toward daybreak given upstream
observational trends over southeast MO/western Illinois.
long term...mesoscale discussion
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