Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
600 am EST Thursday Nov 26 2015
An influx of moisture overnight has allowed showers to overspread
much of Southeast Michigan. The coverage of showers will diminish
during the afternoon as forcing aloft weakens a bit. Ceilings will
remain somewhat variable through the afternoon. Low end VFR and MVFR
conditions will remain more prevalent from ptk northward due to the
closer proximity to higher low level moisture. A little drier low
level air will lift up from the south early this evening...leading
to some improvement in ceiling heights. Mild air this afternoon
should lead to enough mixing to produce some southerly wind gusts
over 20 knots at times.
For dtw...the more widespread showers today will be northwest of
metropolitan. The edge of the more persistent ceilings below 5000 feet will
also reside to the northwest of metropolitan this morning. Some diurnal
enhancements should however support some periodic cloud bases around
4k feet this afternoon.
//Dtw threshold probabilities...
* medium in ceilings at or below 5kft today.
* High that any precipitation will fall as light rain.
Previous discussion...issued 316 am EST Thursday Nov 26 2015
Short term...today and tonight
It will be a very mild start to the Holiday and for some a wet one
as well. Southerly flow has been holding temperatures in the middle to upper
40s overnight...while recent surface observation suggest a steady if not slowly
rising trend to the temperatures this morning. Deep layer southwest flow
will persist across Southeast Michigan today and tonight downstream of a high
amplitude long wave trough. Rain showers have developed over the
central sections of the forecast area overnight along the lead edge
of a middle level Theta-E ridge /850mb dewpoints up to +6c/. A region of
middle level frontogenesis in the 850-700mb layer within this moist
axis is proving efficient in sustaining rain showers. The NAM and arw
both have a reasonable handle on the location of this ascent based
on recent radar trends. They suggest this forcing will persist into
late morning along/north of a line from Ann Arbor to Port
Huron...where rainfall amount of a tenth to a quarter inch look
possible. The coverage of rain showers this morning will decrease
markedly outside of this region of forcing /far southeastern and far
northwestern sections of the forecast area/.
The upstream middle level trough axis will sharpen today...while middle
level heights actually rise across Southeast Michigan. This will weaken the
ongoing region of fgen and actually increase middle level stability...
which should allow the region of steady showers to diminish. A
secondary influx of low-middle level moisture will however target the
Saginaw Valley and northern thumb this afternoon...warranting a continued
mention of scattered showers during the later half of the day /mainly
across the northern sections of the forecast area/. Even with some
showers and ample cloud cover...continued low level warm air
advection under southerly flow will boost afternoon temperatures into the
55 to 60 degree range. Persistent middle level height rises during the
evening and a ribbon of drier middle level air forecast to lift up from
the south suggests mainly dry conditions across Southeast Michigan this evening
/except for perhaps the northern Tri Cities/. The eastward progression of
the northern branch of the middle level trough axis across the northern
plains will drive a surface cold front and associated deep layer fgen
into the Tri Cities/northern thumb late tonight...leading to widespread
rain in these regions /mainly after 09z/. Nighttime temperatures will again
drop only slightly off daytime highs over the forecast area which
will remain on the warm side of the front through most of the night.
Distantly positive tilted upper level trough over central Canada
extending southwest into northern rockies early this morning...with
separate cutoff upper level low over Great Basin. 00z models
continue with slow pace of the northern stream upper level trough
and associated cold front pushing through the central Great Lakes on
Friday...with main upper level energy/pv sliding through the
straights/Northern Lake Huron early Friday evening...with just
modest height falls occurring over Southern Lower Michigan. Even
so...looks like frontal boundary will get just enough of a push to
the south...clearing the state around 00z Saturday...with confluent
flow/subsidence/very dry air (100-500 mb mean relative humidity 20 percent or
lower) taking hold Saturday night. Slow moving nature of the front
during Friday coupled with high moisture content (850 dew points of 5-8
c) expecting quantitative precipitation forecast amounts at least around .25 inch...with potential
of half an inch. Could see enough cold air sneak in underneath late
in the day to support precipitation ending as snow (little if any
accumulation)...especially if any bleed over of precipitation occurs
after 00z...mainly across the south...as the 00z NAM is indicating
low level fgen flare up after midnight right near the Ohio/Michigan
border...but will carry just slight chance for this activity...as
more weight is put on the agressive dry push of the 00z GFS/Canadian
solutions. Amount of clouds leftover on Saturday will ultimately
dictate highs...but around 40 degrees seems reasonable most
locations...per blend of mav/met guidance. Surface high pressure
(1028-1030 mb) in place Saturday night into Sunday...with wave of
low pressure expected to track close to the Ohio River...just far
enough south to keep precipitation chances south of the border. Mins
in the 20s Saturday night...but good radiators could sneak into the
teens if skies are clear through the night. Forecast for Sunday will
feature mostly sunny...but would not be surprised if high clouds
streaming up from south end up being a bit thicker...with just some
filtered sunshine...holding maxes around 40 degrees once again.
Southwest winds mostly around 20 knots today ahead of a cold
front...with enough of westerly component to keep larger waves away
from the nearshore waters...and have dropped small craft advisories
with this forecast package. Expect a long duration rain event today
into Friday as cold front slowly sinks south through central Great
Lakes. Winds shifting to the north behind the front on
Friday....with gusts between 20 and 25 knots to perhaps as high as
30 knots...especially over Southern Lake Huron with longer fetch
into Friday night. Small craft advisories will likely be reissued as
higher waves likely impact nearshore waters of Southern Lake Huron.
Surface high pressure building over the Great Lakes region will
provide lighter winds and low waves over the weekend.
Lake Huron...Small Craft Advisory until 10 PM EST this evening for lhz421.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
You can obtain your latest National Weather Service forecasts online
at www.Weather.Gov/Detroit (all lower case).