Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 221745
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1245 PM EST Wed Nov 22 2017


.AVIATION...

High pressure building into the region will result in weakening
winds and decreasing cloud trends. Northwest flow will back to
southwest as the ridge slides further east into the area tonight.
Meanwhile wind speeds will relax to less than 5 knots over night.
Clouds are the biggest concern as low VFR stratocu eventually
succumbs to the incoming dry air and subsidence this evening. Though
low clouds will pretty much scour out by tonight, a shortwave will
track over the area tonight bringing a period of mid cloud tonight
into Thursday morning.

//DTW THRESHOLD PROBABILITIES...

* Medium for ceiling 5000 feet or less through the late afternoon.
  Low through this evening.

&&

.PREV DISCUSSION...
Issued at 300 AM EST Wed Nov 22 2017

DISCUSSION...

Temperatures are holding steady near 30 degrees this morning as the
state remains socked in with lake effect clouds within northwest
flow regime. Passing shortwave is providing just enough lift to
generate flurries over the area as well as some modest banding
extending into Tuscola, Lapeer, and parts of Oakland/Macomb counties
from Saginaw Bay. Introduced chc pops given that a couple tenths of
accumulation will be possible within this corridor.

Main forecast issue is very low confidence cloud trends today. Given
satellite trends and existing synoptic support over the area for the
next several hours, confidence is high enough to raise cloud cover
substantially through the early morning. As the mid-level wave exits
east in the 12-15z period, suspect downsloping will begin to take
more of a toll over the Saginaw Valley and Thumb areas. Building
high pressure from the west will contribute to backing boundary
layer flow working southeastward from Lake Superior during the late
morning, perhaps sufficiently interrupting the Superior-Michigan
connection enough to allow for additional mid-day cloud erosion.
Developing westerly flow over Lake Michigan will likely force clouds
back into western and northern portions of the area during the
afternoon. Westerly flow will trend WSWly as low amplitude
approaches within NW flow aloft. Even so, expect the bulk of clouds
will pull back toward Lake Michigan tonight as dry advection forces
stable 0-1km theta-e lapse rates over all but the far east quarter
of the lake. This potential for evening clearing and surface high
pressure centered south of the area may allow for some early
radiational cooling. However, temps will quickly level off or slowly
rise for the remainder of the night as the SW gradient ramps up
ahead of the incoming surface trough. Antecedent dry column will
make any snow hard to come by on Thursday morning. The exception may
be the Saginaw Valley area where forcing will be a bit stronger and
some modest lake enhancement to low-level moisture field will be
possible. In any case, there is no expectation for accumulation.

Background height rises will contribute to moderating temperatures
Thursday and especially on Friday as NWP continue to indicate a deep
low tracking north of Lake Superior. Temps will approach or eclipse
50 degrees as the strengthening gradient forces sustained winds of
15 to 20 kts Friday aftn into Fri night. With no gulf moisture feed,
there is a decent chance that the attendant cold front and attendant
LLJ pass through precipitation-free on Friday night.

MARINE...

Moderately active weather during the end of week and weekend as a
number of frontal boundaries push directly through southeastern
Michigan. Additionally, model data is suggesting that a strong low
pressure system will track west to east across northern Ontario
Friday and Friday night. This low pressure system, will likely lead
to increased gradient flow and strong southwesterlies Friday,
veering northwesterly by Saturday. Small craft advisories remain in
effect for nearshore waters off of the Thumb due to cold advection
supported northwest winds. Marine headlines are likely in the coming
days. Small craft advisories will be likely for southwest flow on
Thursday. Gale warnings, 35 to 40 knot gales, will then be likely
for a good chunk of the Lake Huron basin as the pressure gradient
tightens in response to the passing low pressure system.

PREV DISCUSSION...
Issued at 257 PM EST Tue Nov 21 2017

Discussion...

Cold front will surge through area late this afternoon/early this
evening. Lift from upper trough forcing this front aided to some
degree by right entrance region of H25 jet streak lifting up east
side of the trough will maintain a band of rain showers along and
immediately behind this front. Cold air advection will be strong
enough to allow for some of this precipitation to change to snow,
either mixed with the rain or becoming snow showers outright. With
generally light precipitation rates and boundary layer temperatures
in the mid to upper 30s, do not expect any accumulations before the
precipitation ends abruptly in the 22z-02z time frame from north to
south across the forecast area.

Colder air will settle into the area behind this front, with lows in
the low/mid 20s tonight and highs no better than the middle 30s on
Wednesday as the core of this cold airmass settles over the central
Great Lakes. That said, winds will decrease notably as compared to
the past few days, so the "chill" factor will not be as persistent.

A moderation in temperatures will ensue from Wednesday night into
Thursday night as main upper troughing progresses off to the east.
This process will initially be rather slow as another (weaker)
shortwave pivots into the area and maintains some degree of upper
troughing. This should lead to a progression of temperatures from the
middle 20s once again Wednesday night, rebounding to upper 30s to
around 40 Thursday afternoon, but falling back into the mid/upper 20s
Thursday night.

Precipitation, after early this evening, will be basically nil with
perhaps a small chance of snow showers again for at least portions
of the area late Thursday night as a stronger warm air advection
pattern begins to set up in advance of the next strong shortwave
that will be dropping out of the Gulf/Alaska region mid/late week.

MARINE...

A cold front sweeping across the region will bring gusty northwest
winds across the eastern lakes in it`s wake this evening through
tonight. The switch from southwest flow to northwest flow has
already begun as the front currently resides over central Lake Huron
and Saginaw Bay. The front will pass south of Lake Erie later this
evening with a secondary trough and cold surge lagging a few hours
behind. Cold air funneling into the region with a tight pressure
gradient will lead to a period of northwest gales across northern
Lake Huron through most of the overnight period. Slightly weaker
winds across southern Huron will not necessitate gales, but small
craft advisories are in effect for a portion of the nearshore waters
around the Thumb mainly due to elevated wave heights. Lake-effect
snow showers will be common in the cold air behind the front. Winds
will relax early Wednesday morning as high pressure builds into the
region.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory until 1 PM EST this afternoon for LHZ441-442.

Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

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


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