NWS Forecast Discussion

457
FXUS63 KDTX 052322
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
622 PM EST Thu Dec 5 2019


.AVIATION...

Advancing low pressure system across the Midwest has allowed low-
level flow to back to the southwest this evening, which has
maintained dry low-levels through the column. As a result, VFR
conditions are expected to persist through the start of the TAF
period. A series of frontal boundaries will then move across the TAF
sites tonight and overnight caused by the passing low pressure
system, which will maintain the mid-level cloud deck and will result
in the quick veering of surface winds from southeast to northwest.
The area of interest for this TAF period will be the passage of the
cold front, set to sweep across the terminals from 06Z-14Z which will
result in the gradual lowering of cloud bases down to MVFR. A brief
gust around 20 knots will be possible along the leading edge of the
front, with better chances to see more persistent gusts around 20
knots during the daylight hours as post-frontal cold air increases
mixing depths aloft. A dry airmass to then quickly fill in behind the
cold front which will act to eventually scour out cloud cover later
in the day. Went with a more pessimistic, late clearing out time
compared to some models as northwesterly flow will likely tap into
Lake Michigan moisture which would allow residual moisture to hold
under an inversion. Last, removed mention of snow from MBS site as
latest guidance holds bulk of precipitation to the northeast of the
terminal site.

DTW THRESHOLD PROBABILITIES...

* Low for ceilings aob 5000ft tonight, moderate overnight.

&&

.PREV DISCUSSION...
Issued at 351 PM EST Thu Dec 5 2019

DISCUSSION...

Clouds on the increase through the afternoon as mid level moisture
advects in ahead of a clipper that will sweep across Mid/Northern MI
tonight. A dry layer currently between 3-8kft will take a bit to
overcome as evident in upstream obs failing to report any precip
hitting the ground under the increasing coverage of radar returns.
The clipper will strengthen while passing over lower MI with the
leading arm of the warm front igniting some light snow showers
around 03Z after saturation has occurred. This initial activity will
fall over northern parts of the Saginaw Valley and Thumb with some
expansion southward as the low draws nearer. Highest confidence is
placed over the Thumb which has the highest amount of QPF, the
coldest air and best mid level forcing. Lower confidence for both
POPs and thus any accumulations (of rain or snow) farther south as
soundings show the dry air holding firm south of about I69.
Soundings have also been consistent in the picture they paint of the
thermal profile adding some question to ptype. A deep moist layer
from the sfc to 12kft is advertised by the NAM, GFS, RAP with only
the very top of this layer tapping into the air at or below -10C and
weak lapse rates throughout the column. As the night goes on the
depth of the moisture decreases while losing connection with this
snow growth zone making a rain/snow mix more likely. Locations
around I69 or south may get all rain but with surface temps very
near 32, don`t expect too much freezing to occur. Models have been
trending a bit higher with moisture content and strength of the fgen
band with the best forcing being located just to the north of SE MI.
Have made minor adjustments to push back timing slightly and
increase potential snow totals over Huron County to around an inch,
possibly up to 2 inches if the best forcing and moisture (closer to
0.2 inches) can shift south slightly. Otherwise most locations north
of I69 should see more like a half inch with some mix of rain and
snow.

Northwest flow sets up for Friday as the clipper pulls off to the
east and longwave ridging builds across the central conus. Area of
surface high pressure will encompass the Great Lakes leading to a
cold (highs in the mid 30s) but dry day. Subsidence and dry air
could bring about a good deal of sun through the afternoon. Cold
temperatures hold through Saturday but the surface ridge axis passes
during the day which flips the flow to warmer southwest flow. It
will take til Sunday for temps to respond as the thermal ridge folds
into the region, but highs will then jump to the mid/upper 40s for
both Sunday and Monday.

Next trough will start to push south into the northern Plains and
Midwest Sunday night. The initial low over northern Ontario will lay
out a front that will push south through the Great Lakes Sunday
night. Meanwhile a strong southern stream jet working through the
central Plains will result in the development of a low that will
then strengthen while lifting northeast along the front through
lower MI. Warmth of the resident airmass and track of the low
through Mid MI will result in a rain event across SE MI Monday and
Monday night, with a chance to switch to snow on the tail end behind
the cold front overnight. An arctic front will quickly follow on
Tuesday ushering in a cold airmass with 850mb temps around -20C. The
tight westerly gradient on the backside of the exiting low and surge
of cold air should be enough to excite lake effect snow off Lake MI
in which would last through the day Tuesday.

The arctic air and surface high pressure drifting across the Ohio
Valley mid week could lead to daytime highs Wednesday only in the
low 20s, dipping into the low teens Wednesday night. The coldest air
should then pull away by Thursday allowing temps to slowly
moderate through the end of the week.

MARINE...

Period of light southerly wind as a weak low transits the waters
tonight will be brief as it exits east to New England by Friday
morning. In its wake, strong northwest wind will develop and gust to
near-gales again on Friday afternoon. The prolonged northwest fetch
will likely warrant small craft advisories for elevated waves along
the exposed nearshore waters of the Thumb. Flow then veers to
southwesterly as warm air floods into the Great Lakes region late
Saturday into Monday, gust potential limited by the stable southerly
fetch. Conditions become more unsettled for the remainder of next
week as the pattern becomes more active and colder air filters into
the region.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

AVIATION.....99
DISCUSSION...DRK
MARINE.......JVC


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NWS DTX Office Area Forecast Discussion