Forecast discussion - through Tonight

The beginning stages of the active upper level trough will commence later this evening as the
isentropic ascent along the elevated warm front starts to ignite over the area. An initial ribbon of
moisture is currently working northward but with deep layer of dry in the lowest 10Kft of the
column, this moisture is only acting to prime the upper levels for the deeper plume of moisture
further south and west. This enhanced plume on the nose of the 850mb theta-e gradient will lift
north through the area between about 03-06z. Deep layer of southerly flow will lead to strong warm
air advection with 900mb temps approaching 5C by 00z tues. Warming continues with temps above 0C
extending up to 800mb. Temps at the sfc will be slower to climb as the sfc warm front remains to our
south through the overnight. This will mainly be a non issue with dewpoints climbing into the
low/mid 30s and warm air aloft, precip should fall as rain for the majority of se Michigan. The area
of concern for some frozen form of precip will up across our northern counties, parts of
Saginaw/midland/bay. This area will possibly see sfc temps hold aob freezing at the onset of the
precip which produces a thermal profile which could result in some light freezing drizzle. If the
rainfall rates get high enough at the start it may decrease chances of refreezing on cold surfaces.
At this point, will go with a light rain or freezing drizzle forecast as it is not a sure thing by
any means and will then monitor obs this evening and handle accordingly.

.

Remaining Forecast discussion -

Focus continues to be on impacts associated with strong cyclogenesis Christmas eve. Strong upper jet
/e.g. 175 kt sampled by spokane Washington state 12z raob/ will amplify the central Contiguous U.S.
longwave trough already in place. Weaker upper wave will shear out while associated surface
reflection dissipates on Tuesday, giving way to stronger energy rounding the base of the trough.
This energy will lift northward, becoming negatively tilted and closing off Wednesday. Associated
surface cyclogenesis will take advantage of preexisting baroclinic zone, deepening around 20 mb from
06z wed-06z Thursday as it lifts from the deep south toward Lake Huron.

Models generally in good agreement through about 06z Wednesday with initial wave and accompanying
moisture surge. Expect an area of rain showers associated with a warm front to be ongoing at 12z
Tuesday and translating newd across the local area. This shower pattern should diminish from sw to
ne from the late morning into the afternoon as we briefly get into the warm sector of the decaying
initial cyclone and deep-layer moisture decreases. As such, tapered pops to chance by afternoon.
Could still be some areas of drizzle around though with plentiful lower clouds. Mild high temps in
the 40s. After 06z wed, well-advertised model differences in the secondary /stronger/ surface low
have caused headaches and uncertainty for previous forecast packages. The good news is, at least for
the 12z model suite, a consensus appears to be emerging. The consensus track, typified by the 12z
GFS/ECMWF/ggem, takes the strengthening surface low from around nashville Tennessee at 12z wed,
northward to Lake Huron by around 06z thu, straight through or just west of DTX cwa. The 12z ECMWF
especially increases confidence in this warm solution as it has trended toward the GFS. Basically
disregarded the 12z NAM eastward cold outlier. Despite this emerging consensus, there are still
differences in the timing and strength of the surface low.

Gulf moisture will re-enter the region late Tuesday night with 850 mb dewpoints increasing to 7-10C
during the day Wednesday and precipitable waters around 1 inch per the GFS. Lift will increase as
well due to system-relative isentropic ascent, fgen forcing with baroclinic zone tightening with
time, and cyclonic vorticity advection with deepening negatively- tilted upper wave. The result will
be a wet and mild Christmas eve. With the track of the low-level cyclone over or to the west of our
region, precip will be solidly liquid during the day.

After 00z Thursday any precip will change to snow as the low moves off to the north and colder air
comes in on the southern flank of the low. The question is, how much forcing/moisture will be
available on the back side of the low. 12z suite has cast doubt on snow potential with a dry slot
over much of se Michigan and the main deformation snow just to our north and west. However, with
well- publicized model consistency issues, will continue with the 1 to 3 inch idea until we can get
a little more run-to-run consistency. A little eastward shift is definitely not out of the question,
which would put the deformation snow over se Michigan. Regardless, low temps should still be rather
mild, mostly around freezing as the cold air on the backside of this system is not all that cold,
with h85 temps around -6. Other story will be the winds, with the low still deepening to our north.
Kept the idea of 35 mph wind gusts Wednesday night.

Kept dry forecast Christmas day as aforementioned cold sector air that is not actually that cold
should not cause much of a Lake Michigan response, and we actually get h85 warm advection later in
the day. Rather breezy conditions can be expected much of the day, though winds will decrease with
the diminishing pressure gradient through the day.

. . posted at 337 PM EST Monday December 22 2014


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