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fxus63 kdtx 200455 
afddtx 


Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan 
1255 am EDT Sat Mar 20 2010 




Aviation... 


The cold front will be advancing south of the Detroit terminals near 
the start of the 06z taf issuance. Post frontal moisture will 
steadily increase during the morning...leading to widespread MVFR 
ceilings. A mix of light rain and snow is expected develop and 
change over to all snow during the course of the morning at 
ptk...fnt and mbs. Conditions will improve from north to south 
during the afternoon and evening as drier air advects in under the 
persistent northerly flow. 


&& 


Previous discussion...issued 909 PM EDT Friday Mar 19 2010 


Update... 


As of 01z...the surface cold front is advancing toward the I 69 
corridor /already south of phn thanks to the cold waters of Lake 
Huron/. This front will continue to push south through the 
remainder of the night /expected to move south of Adrian shortly 
after 3 am/. The passage of the front will be noted by a sharp 
shift in the winds to a more northerly direction and a good Ten 
Degree drop in temperatures. Regional radar show an axis of 
precipitation extending from Kansas into northwest lower Michigan. This precipitation 
is occurring within an elevated frontal boundary and along an axis 
of deeper moisture...not forecast to advance into the Tri Cities 
and northern thumb until late tonight. Thus have pushed back the 
timing of the precipitation. Upstream observations and degree of frontal 
forcing will warrant raising probability of precipitation across the Tri Cities late. The 
deeper moisture and forcing however will not work any farther 
south until after 12z. Thus have backed off on probability of precipitation south of fnt. 


Precipitation will begin as very light rain...but will transition to 
light snow during the course of the night as the depth of the cold 
air increases. 


Previous discussion...issued 403 PM EDT Friday Mar 19 2010 


Short term...tonight 


Today will mark a temporary end to quiet weather and well above 
normal high temperatures as a cold front lurking over northern 
Michigan heads for Southeast Michigan. 


Frontal passage is expected to occur this evening per latest 
guidance from NAM/GFS. Although skies are fairly clear out ahead of 
this feature...satellite imagery confirms low/middle level moisture 
riding up along and behind the boundary. Will see a gradual increase 
in cloud cover from north to south...with mostly cloudy skies across 
just about all of the forecast area by midnight. Good 
frontogenetical forcing and right entrance region dynamics should be 
able to overcome resident dry airmass as the front pushes 
through...allowing for scattered coverage of rain and snow showers 
tonight. This will especially be true after 06z...when additional 
low/middle level moisture is expected to ride up the 925-850 mb frontal 
boundary...which will slow then stall over the area by 12z/8am. GFS 
looks to have a better handle on upstream quantitative precipitation forecast/precipitation and have 
leaned towards this model for the forecast. Much colder air will 
spill into the region behind the front...allowing temperatures to 
fall into the 30s tonight...with a few locations even dropping into 
the upper 20s over The Thumb and across Bay and Midland counties. 
Strong cold air advection combined with evaporative cooling at the 
onset of precipitation will allow for a north to south changeover to 
snow...with a chance for mixed precipitation reaching as far south 
as the city of Detroit by sunrise. With the very warm ground and 
only light precipitation...do not expect accumulations outside of 
perhaps a tenth north of Saginaw. 


Long term...Saturday through next Friday 


The cold front will be through all of Southeast Michigan by sunrise 
Saturday morning, at least in terms of the wind shift. Cold air will 
then build in depth over the region throughout the day, but will 
already be deep enough for all snow in the Tri Cities and thumb. 
This is judging by 1000-850mb thickness sub 1290 by 12z in all of 
the models which is supported by temperature observations in the 
upper 20s this afternoon throughout Minnesota and reports of snow as 
far south as central Iowa. We did allow daytime highs to rebound a 
few degrees from tonight's lows but the feeling is even a small 
diurnal range is optimistic. 


With confidence in the presence and depth of cold air, the question 
then is one of precipitation coverage and amount. A high coverage 
and low quantitative precipitation forecast scenario remains expected but leaned this package toward 
the higher end of model quantitative precipitation forecast offered by the GFS. This is based on 
observations in Iowa this afternoon that generally support the GFS 
over the lower amounts in the other model solutions. Satellite 
imagery also indicates some texture in the clouds which suggests 
good middle level instability with increasing moisture, and the 
frontal circulation is not even as fully developed as it will be 
by Saturday afternoon. The one thing the models do have in common 
is the depiction of an increasingly favorable right entrance 
region of the upper jet over lower Michigan. This feature is 
expected to drive an increasingly strong ageostrophic circulation 
and support middle level frontogenesis over Southeast Michigan during the 
afternoon and into Saturday night. There is a noticeable 
tightening of Theta-E in the 850 to 700 mb layer that advertises 
the fgen forcing and model cross sections do show some elevated 
instability/layers of low stability to enhance vertical motion. 
However, the frontal surface is on the shallow side and space 
sections do not show any particularly active layer, negative 
factors that support keeping probability high but with modest 
liquid amounts and with any snow accumulation confined to unpaved 
surfaces. Snow accumulation will be limited by moderate 
precipitation rate occurring during the afternoon hours in late 
March, and by warm ground conditions after this week's stretch of 
above normal temperatures. 


This system will linger through Sunday with middle level portions of 
the front remaining overhead but the upper jet support will steadily 
weaken with time. Expect this to result in a trend toward just a 
chance of light rain by afternoon into Sunday night and then just 
over south sections of the area as the front and Ohio Valley low 
move steadily eastward. Trends in the 12z model package show the 
Ohio Valley low tracking farther south but about on the same 
eastward pace as previous runs. This will allow US to remove 
precipitation from the forecast by Monday afternoon with dry 
conditions leading to a temperature rebound, at least for inland 
areas, by Tuesday. Highs in the middle 50s by then are once again 
about 10 degrees above climatology. 


Cautiously optimistic that Wednesday will be another mild day in Southeast 
Michigan before the next cold front arrives. Predictability is 
low by then considering the front is associated with a short wave 
within the broad upper low over central Canada. The latest global 
model solutions exhibit considerable spread in timing the front 
through our area, but all exhibit split flow tendencies in the 
larger scale upper level pattern which favors a faster northern 
stream short wave and associated frontal passage on Wednesday. This 
supports maintaining a chance of rain in the forecast for Wednesday 
into Wednesday night and keeping temperatures close to guidance in 
the lower to middle 50s, subject to adjustments based on timing changes 
to the frontal passage. If adjustments are needed, they would likely 
be toward slower timing and/or less southward progress of the front 
heading into the end of next week, again depending on the eventual 
evolution of the split flow pattern aloft. The European model (ecmwf) is the most 
aggressive on a repeat performance of southern stream low pressure 
development along the front in the southern Great Lakes/Ohio Valley 
and has support from the Canadian, the fim, and a lesser extent the 
GFS. This would result in a return to below normal temperatures and 
a chance of rain and snow for Thursday and Friday. 


Marine... 


Southwesterly wind gusts to 22 knots have developed close to the 
land areas this afternoon ahead of an approaching cold front. These 
gusts will subside around sunset. The cold front will push down 
across the central Great Lakes this evening and overnight... 
bringing a resurgence of winds...but this time from the north. 
Wind gusts over the nearshore areas are not expected to exceed 20 
knots...however sustained north winds over the length of Lake 
Huron will allow waves to build in excess of 4 feet from Port 
Austin to Port Sanilac late tonight and into tomorrow afternoon. A 
Small Craft Advisory is therefore in effect. 


A ridge of high pressure will briefly extend into the northern Great 
Lakes tomorrow behind the front...before two low pressure system 
move into the region on Sunday. The first will track through the 
northern Great Lakes on Sunday...while the other lifts across the 
southern Ohio Valley Sunday night. This will bring unsettled weather 
to the southern Great Lakes into the beginning of next week. This 
will also allow northeast winds to increase across all marine 
areas on Monday. 


&& 


DTX watches/warnings/advisories... 
Michigan...none. 


Lake Huron... 
Small Craft Advisory...nearshore waters from Port Austin to Port 
Sanilac...from 5 am Saturday to 4 PM Saturday. 


Lake St Clair...none. 
Michigan waters of Lake Erie...none. 


&& 


$$ 


Aviation.....Kurimski 
update.......SC 
short term...hlo 
long term....bt 
marine.......hlo 




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