Scientific Forecaster Discussion
Area forecast discussion
National Weather Service northern Indiana
1235 PM EST Monday Dec 22 2014
issued at 1127 am EST Monday Dec 22 2014
Low pressure moving through the north central United States will
be giving the region some rain later today through tonight. Could
have a break in the rain tomorrow before a low moves up from the
south. This second low is expected to bring first rain and then
snow on Wednesday...with snow chances continuing into Wednesday
night. High temperatures today will range from around 40 in
southern Michigan to the lower 50s in southeastern Indiana. Lows
tonight will range from the upper 30s in southern Michigan to the
lower 40s near the Ohio River.
Short term...(today and tonight)
issued at 400 am EST Monday Dec 22 2014
An impressive 170 knots east Pacific upper jet nosing into the Pacific
northwest/rockies will continue to help amplify a downstream central Continental U.S.
Longwave trough today into tonight. 00z models in good agreement in
this jet energy carving out a deep closed circulation in vicinity of northern
MO/Iowa by tonight. Warm/moist conveyor belt in deepening south-southwest flow
and increasing diffluence aloft east of associated height falls
will work into the area later today-tonight bringing good chances
for rain along and in advance of an occluded front. Increasing
probability of precipitation for light rain (highest west of I-69) this afternoon/evening will
be tied to the initial warm advection/isentropic surge (3-4 g/kg
push within 30-40 kts of cross isobaric flow on 290-295k sfc's).
Low wet bulb zero heights a bit concerning for a brief period of
light freezing rain at onset of precipitation northern half of forecast area (mainly
northeast zones...or possibly some light fzdz western zones later
this morning. However...probability of occurrence remains too low
to include in the forecast with bulk of guidance quickly warming temperatures
above freezing. Deep uvm/moisture plume with precipitable waters nearing 0.80
inches within Theta-E ridge overspreads the local from SW to NE
overnight. A 3-6 hour period of more moderate rain shower activity
will result with categorical probability of precipitation warranted.
Long term...(tuesday through sunday)
issued at 400 am EST Monday Dec 22 2014
Not really a surprise that models have come back to a version of
their previous solutions with regards to the upcoming winter storm
this week. While uncertainty continues it is likely that with the
initial sampling of the jet energy over the Pacific northwest by radiosonde observation
network may finally be helping models hone in on expected large
scale evolution. Rather good large scale model agreement now with
current run and for most part previous run. However...this is by far
a lock with numerous questions remaining regarding critical thermal
details and important exact track of actual low pressure centers.
Long term period begins Tuesday with questions developing with
respect to how much measurable precipitation will be around between systems.
Latest GFS showing a rather pronounced dry slot invading the area
early behind departing first wave of moisture as initial low
occludes and begins to fill. 00z European model (ecmwf) hinting at this as well but
does not appear to be as pronounced. Abundant low level residual
moisture will be left across the area and numerous subtle short
waves and vorticity centers being shown along with pronounced deep
southerly flow. Expect this to lock low clouds in with either
drizzle or light showers possible through the day. Thus will leave
rather high probability of precipitation in forecast for now closer to ece guidance but
actual precipitation amounts will likely be on the lighter side Tuesday into Tuesday
The strong Pacific jet energy will dive south and carve out a large
scale trough over the central Continental U.S. By Tuesday. This will lead to
cyclogenesis near the Gulf Coast or even over the Gomex Tuesday.
This low will deepen as it moves almost due north into the Ohio
Valley and upper Great Lakes on Wednesday. This will spread a good
deal of rain back across the area with amounts upwards of an inch
possible Wednesday. The critical exact tracks of the middle and low
level low pressure centers remain in question as 850-500mb lows
close off. While spread in 00z models has narrowed
considerably...minus the weak nam12 outlier...the spread is still
large enough to keep uncertainty high with respect to timing and
location of changeover from rain to snow and resultant amounts.
Preference still lies with the western most solutions as we have
stated previously tends to verify best with strong dynamical and
negatively tilted winter storms. The parallel GFS and gefs means are
similar in this idea...taking surface low from near sdf to kfwa Wednesday.
However...the degree of negative tilt this system takes and the
timing of the phasing still vary amongst the models and will
ultimately decide the track. Regardless of the various
tracks...current indications still support a later changeover to
snow Wednesday afternoon. With a very wet and relatively warm ground
from rain it will be difficult for snow to accumulate initially. The
caveat here is the degree of dynamical forcing and cooling near the
track of the 700mb low and strength of deformation zone on the west
side of the low track. Expect with this rapidly intensifying system
that we will see an impressive trowal take shape. Thermal profiles
taken verbatim on current runs would support mainly rain with a wet
snow mix Wednesday afternoon changing over to all snow in the evening
before ending overnight.
Expected accumulations remain a long shot and still highly
uncertain. However...we are nearing the time period for inclusion
within grids so best estimates at this time support a swath of 1 to
3 inches on back side. Again...very early and low confidence
prediction. Those with travel plans or concerns should continue to
monitor updated forecasts through the early part of the week leading
up to the storm.
Several clipper systems to follow for late week into next weekend.
Model chaos with initial big system leads to little predictability and
confidence in later periods with these fast moving systems. It does
appear a return to more winter like temperatures is on the horizon
for the week 2 period with Arctic air poised to move south.
Aviation...(for the 18z tafs through 18z Tuesday afternoon)
issued at 1212 PM EST Monday Dec 22 2014
Warm front over the Ohio Valley will lift slowly north-NE causing
deteriorating aviation conditions as rain overspreads the area
this aftn/eve... continuing tonight... then transitioning to dz Tuesday
morning as the front moves north of the area. Current VFR
conditions across the area should lower to IFR this evening with LIFR
possible late tonight and Tuesday morning.
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