Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
131 PM EDT Sat Apr 18 2015
VFR sky conditions are forecasted for the duration of the taf
period. Northeasterly winds of 15 to 20 knots have developed as
forecasted and will persist throughout the remainder of the
afternoon hours. No rain is expected between 12-18z Sunday with
model data trending slower with the onset of precipitation.
//Dtw threshold probabilities...
Previous discussion...issued 344 am EDT Sat Apr 18 2015
Backdoor cold front will settle through the County Warning Area during the day
today. High pressure centered over Ontario will drive cool northeast
flow across the area. Locally, lake enhancement of synoptic scale
high pressure will ensure a healthy cool push of marine-modified air
from northeast to southwest during the daytime hours. Thus, while
locations from near Ann Arbor and points south may touch 70,
temperatures will be stuck in the 60s elsewhere...40s near the
shoreline in The Thumb. Insolation within this regime will deepen
the boundary layer favoring some mixing up to 900mb and afternoon
wind gusts to 20 knots. Deep layer subsidence downstream of the
amplifying shortwave ridge over the upper MS valley will clear the
column of clouds for the balance of the day save for some afternoon
Low pressure lifting into the lower Ohio Valley will continue to
spread the blanket of cirrus overhead tonight while also ensuring
the boundary layer remains relatively well-mixed courtesy of the
Stout easterly gradient. The nam12 seems to have a general handle on
low temperatures south of M59 with lows generally 40 or higher.
However, negative radiating appears way too strong in The Thumb and
Saginaw Valley given the magnitude of the expected pressure
gradient. Thus, leaned toward guidance that depicts a more uniformly
distributed and warmer temperature field with minimal local effects.
Long term...Sunday through Friday
The extended period will be pretty gloomy with plenty of cloud cover
and chances of rain as a large area of low pressure aloft moves ever
so slowly through the Great Lakes.
The longwave pattern will be transitioning from split flow with a
cutoff low in between...to that of of a deeply troughed northern
stream which merges with the southern stream over the eastern Continental U.S..
the trough Will Park itself over the Great Lakes through the better
part of the coming work week. As is typical of cutoff
systems...models are fluctuating a bit with timing of shortwave
features and better plumes of moisture. This is resulting in a
chance of rain nearly ever period through middle week though there will
likely be a dry period or two in there.
The first good surge of moisture will be Sunday afternoon as the
upper low begins drawing moisture up into the region from the Gulf
with a deep layer of southerly flow extending up through the column
ahead of the system. Initially...easterly flow out of Canada will
keep dry air in the bl which will delay the onset of rain over Southeast
Michigan. But a good surge of Theta-E after 18z Sunday which will be
enough to overcome the dry air. Likely probability of precipitation will take US through 00z
Monday which is when the surface low from the southern stream lifts up into
the region bringing an even more saturated airmass. Precipitable waters over an
inch Sunday afternoon will reach near 1.5 inches Sunday night. Went
categorical for the overnight period to account for this ribbon of
moisture and enhanced forcing tied to the low itself.
The low will lift north-northeast through lower Michigan before pulling a cold front
through southern Michigan on Monday. Models are less impressive with
forcing and instability with the cold front with the latest runs.
Column appears too saturated and warm too far up the column to
result in much instability. But it is a cold front and with the
models still adjusting each run due to the speed of the cutoff
low...do not want to pull thunder mention out just yet so will leave
a slight chance mention in at this time.
Tuesday and beyond...troughing at all levels is slow to leave the
region sending shortwaves and lobes of vorticity through the region
through midweek. Timing these features will come later on so will go
with a low chance pop through about Thursday which is when it looks
like high pressure will make its return. Besides on and off shower
chances...the forecast will lean toward the cooler side as 850mb
temperatures drop into the negatives behind the cold front...making a run
at -10c Wednesday and Thursday. This will make it challenging for highs to
reach 50 degrees later this week while lows dip to below freezing at
Cold frontal passage on Friday has resulted in northerly winds over
the eastern lakes. Winds will increase while becoming northeast
early this afternoon with some channeling into Saginaw Bay. This
will lead to elevated winds and waves over the Bay resulting in a
Small Craft Advisory through this evening. Winds and waves will
briefly relax as high pressure builds into Michigan and Ontario
overnight. A low pressure system will move into the area on Sunday
causing winds to increase once again out of the east before veering
to the southeast. The easterly fetch with increased flow will cause
waves to build in the nearshore zones likely necessitating small
craft advisories for the nearshore areas Sunday into Monday. Gale
force winds are possible but not likely at this time.
Lake Huron...Small Craft Advisory until 8 PM EDT this evening for lhz421-422-
Lake St Clair...none.
Michigan waters of Lake Erie...none.
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