Area Forecast Discussion

FXUS63 KDTX 182026
Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
326 PM EST Sun Nov 18 2018
Upper level jet entrance region dynamics responsible in generating 
narrow swath of light snow today for locations south of I 96 has 
exited to the east. Thus, synoptic scale forcing for ascent has 
ended for today. 
For tonight, weather across Southeast Michigan will be driven by a 
weak ridge of surface high pressure that will build directly over 
the cwa. The surface ridge appears to be delicate and wedged between 
a developing pseudo split flow aloft. Models do show some positive 
AVA, although rather inconsequential. The main forecast question for 
tonight is how progressive cloud clearing will be. Survey of 
vertical moisture plan view progs support a deep layer of dry air 
for the overnight. Model soundings do show sharp subsidence 
inversion base down to around 2.5 kft agl. No shot at any tangible 
advection overnight with neutral type flow with respect to 
isentropic surfaces. Given the look of satellite imagery with 
boundary layer cumulus/stratocumulus this afternoon and very shallow 
depth to any saturation at the top of the boundary layer, 
preferences is to maintain the inherited forecast of aggressive 
clearing trend late this afternoon and tonight. As a result, did 
blend some cold regional Canadian into the low temperature forecast 
for tonight, ranging lower to mid 20s.
The first of a couple of arctic boundaries will be pushing southward 
through the northern Great Lakes as the developing northern stream 
sets an edge. Combination of some entrance region dynamics to subtle 
jetlet and some diffuse cyclonic vorticity advection will lead to 
light preciptitation to the north of Saginaw Bay through Monday 
afternoon. Center of H7 anticyclone will pass overhead during the 
morning hours but as this becomes increasingly squeezed eastward, 
mid to high clouds on the warm side of approaching frontal boundary 
will lead to an increase in mid-high cloud Monday. Cannot rule out 
some boundary layer cloud growth that may happen late. Message of 
sunshine early with increasing cloud appears apt, highs in the 
middle 30s.
Monday night. The midlevel anticyclone center will be well east of 
Southeast Michigan. This will allow broad, disorganzied cyclonic 
vorticity energy to wash across the state as a very deep midlevel 
trough/associated cyclonic flow at the longwave arrives. Most likely 
of responses will be some Lake Huron aggregate troughing as cold air 
will be settling southward within the composite trough. Modest warm 
advection centered around 600mb and magnitude of differential cva 
will be enough to generate likely light snow PoPs. Main timing 
appears set between Midnight-7AM Tuesday morning. Model soundings 
show a favorable thermodynamic sounding for snow microphysics with 
supersaturation with respect to ice throughout a deep layer. Current 
indications are snow ratios will range between 12:1 to 16:1. Snow 
accumulations of 1 to 2 inches appears possible north of M 59 
conditional to how the atmosphere evolves, snow accumulation of less 
than an inch for Metro Detroit. Higher amounts will be possible in 
the eastern Thumb, in closer proximity to Lake Huron shoreline. 
Onshore flow may lead to some lake effect snow showers throughout a 
portion of the day Tuesday.
A reinforced thermal trough will hold temperatures below normal 
Wednesday into Thursday with h850 temperatures averaging -12C by 00Z 
Thursday. Overcast conditions and caa will cap temperatures in the 
low to upper-30s on Wednesday, with the coldest temperatures seen 
across the Tri-Cities region into the Thumb. An advancing surface 
high pressure system will travel from western Ontario and will 
center east of Lake Huron by Thursday afternoon, providing dry 
weather both Wednesday and Thursday. Increasing subsidence will also 
begin to produce breaks in cloud cover and will bring a mix of 
clouds and sunshine by Thursday, however, the strong caa ahead of 
the high will bring even cooler air into SE MI, lowering daytime 
highs down to the low-30s on Thanksgiving. 
Upper-level troughing is expected to strengthen across the Plains 
late this week and will reinforce a ridge across the Great Lakes and 
northeast U.S. which will act elevate temperatures into the 40s for 
a high Friday into the weekend. Anticylonic flow around high 
pressure will increase h850 temperatures above the freezing mark, 
averaging 4C by 18Z Friday. Increasing surface temperatures in the 
mid-40s are forecasted for the weekend, with a slight chance to peak 
up into the upper-40s across the Metro region. 
Advancing warm front will provide the chance to see precipitation 
starting late Friday and into the weekend. At this time, confidence 
is too low to discern type of precipitation if precip materializes 
during the night and early morning hours, as temperatures fall into 
the mid-30s for overnight lows. As a result, a blend of rain/snow 
mix has been added to forecast until better convergence is noted 
with new model suites.
Moderate southwesterly flow of 20-30 knots will develop today as the 
pressure gradient tightens between the center of high pressure 
located over the central Plains and low pressure tracking across 
southern Hudson Bay. The strongest winds will occur over the 
northern and central Lake Huron basin. Moderate southwesterly flow 
will continue into Monday before a cold front drops south across the 
Great Lakes switching winds post-front to the north with 
accompanying chances for snow showers. A clipper will then track 
southeast across the northern Great Lakes during the midweek period, 
with the potential for northwest winds to gust to marginal gales on 
Issued at 1228 PM EST Sun Nov 18 2018 
Conditions will improve to MVFR early in the period as vsbys improve
in response to waning snowfall, which will end by 18z. Cigs then
further improve to low VFR under a light/variable gradient as an axis
of surface high pressure settles through the area. As modest return
flow emerges beneath the inversion tonight, uncertainty increases
with respect to MVFR filtering back north as indicated to varying
extents by the high res guidance. For now, retained SCT coverage for
most of the night with high pressure axis over the area, but allowed  
a return to BKN toward Monday morning. 
* High for cigs aob 5kft through this evening. Low overnight.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
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