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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
1043 am EDT sun Apr 19 2015


The previous discussion issued with the morning forecast package
outlines the today and tonight periods very well and remains an
outstanding reference.

The main objectives of the recent update was to refine timing of
precipation chances this afternoon and evening and to include a
chance of thunder. There are a few main points to drive home.

The first is concerning the chance of showers and a thunderstorm
this afternoon. Model data supports carrying the current activity
south of Cleveland northwestward. The convection is occuring along
an enhanced gradient of moisture/warm frontal arc that has origins
from the applachian chain. This activity is expected to push
across Lake Erie and arrive over southeastern Michigan between
15-18z. Model data does support some potential for increasing
areal coverage when it arrives. Did introduce a thunder mention
given the trend of kcle reflectivies and nldn lightning reports.
Soundings suggest as much as a few hundred j/kg of MUCAPE with a
pocket of steep lapse rates between 800-700mb. Expect this
conditional activity to push steadily northward throughout the

The Second Point of emphasis is with the timing of the main band
of precipitation now lifting into southern sections of southern
Indiana and southern Ohio. Overwhelming model support exists along
with agreement from radar linear extrapolation time of arrival in
bringing this area of soaking rains across the southern County Warning Area border
right at 21z. Pushed timing of categorical probability of precipitation back a few
hours. A shorter duration of heavy rainfall outlined in recent
discussions certainly remains a possibility for this evening.


Aviation...issued 726 am EDT sun Apr 19 2015

Easterly winds will gust to 20 knots through the daytime hours as
low pressure approaches from the south. A few isolated showers 15-
20z will not be Worth a specific mention in the forecast. Deep
moisture and strong forcing will arrive around 21z bringing rain and
rapidly deteriorating aviation conditions that will persist for the
remainder of the night. IFR visby is likely as rain intensity
increases by 00z in the Detroit metropolitan area with periods of heavy
rain possible through at least 04z. As the precipitation shield lifts
north, gusty southerly winds and showers are expected to fill in
while lingering low-level moisture contributes to continued IFR
through daybreak Monday.

For dtw...showers are forecast to begin at dtw 21-22z with the
target period for potential heavy rain 01z-04z. Easterly winds will
veer to southeasterly around 08z becoming south-southwesterly by 12z

//Dtw threshold probabilities...

* high in ceilings at or below 5kft


Previous discussion...issued 355 am EDT sun Apr 19 2015

Short and tonight

Large expanse of quiet weather stretching from Missouri into
Kentucky at 07z this morning as the forcing associated with the
strong low over the western High Plains has yet to arrive. A partial
coverage of showers is noted in vicinity of Memphis/Paducah within the
Reservoir of high moisture content residing over the Tennessee and
lower Ohio River valleys. The shortwave digging into the northern
plains will cause the strong closed low over Kansas to open up and
translate east over the next 24 hours. The arrival of deep layer
south/southeast flow east of the Mississippi River by 18z will
advect this slug of moisture northward. At the same time,
superposition of increasingly favorable upper atmosphere divergence
and isentropic ascent will force rainfall to Blossom over Kentucky/IL/in
throughout the morning.

Supported in part by a complex of shortwave energy emanating from
convection over the deep south, this activity will lift toward
lower Michigan during the day. Initial arrival of middle-level moisture
coincident with a narrow band of 290-295k ascent may support a few
showers during the late morning-middle afternoon, but lack of
appreciable depth/duration suggests maintaining a conservative
approach to the forecast through this period. As such, held most
places at 30-40 pop with minimal quantitative precipitation forecast through at least 21z. Stiff
easterly flow will put a lid on high temperatures, but still expect
most places not near Lake Shores to approach or exceed 60 degrees,
roughly in line with the 00z mav guidance, as predominantly dry
conditions dominate through late afternoon.

Primary corridor of 305-310k isentropic ascent will lift from south
to north through the forecast area 21z-06z this evening beneath
strongly divergent flow aloft. Low-level jet forcing could be
considerable during at least a portion of this time frame as a 60-
70kt low level jet impinges on the eastern County Warning Area. Strong moisture transport
centered within a deep 10kft warm cloud layer will lend itself to
highly efficient hydrometeor production and several hours of
moderate rain with embedded heavy showers. Favored guidance
indicates a quantitative precipitation forecast maximum along/east of the glacial ridge with a solid
0.6" to 0.8" areal average across the urban corridors of metropolitan
Detroit. Urban and Small Stream flooding will be possible, largely
dependent on nature of low level jet evolution during the lower impact
time period of 02-07z for Detroit and surrounding areas.

Rain will diminish in coverage as the Wing of strong isentropic
ascent and a sizeable percentage of middle-level moisture lifts north.
However, increasingly cyclonic flow will be spreading into lower
Michigan at this time and the abundance of remnant low-level
moisture will be accompanied by steepening lapse rates as the
thermal ridge lifts northeast. As a result, expect a healthy
coverage of showers to continue into the predawn hours Monday
morning. The strong gradient and steep lapse rate environment should
also contribute to a period of 25 to possibly 30 knots wind gusts
before the strong wind field over Southeast Michigan diminishes early

Long term...Monday through Saturday

After a fairly pleasant week or two in Southeast Michigan the long term takes a
turn back to cool and dreary with several opportunities for rain
showers...and possibly a few snowflakes.

By Monday the region will be be engulfed by a large area of surface low
pressure resulting from complex interaction of the northern and
southern streams of the jet in addition to the remnants of a cutoff
low that drifted across the Continental U.S. Over the last few days. The
middle/upper level trough will still be off to our west by Monday
morning with the first strong shortwave lifting through the northern
Great Lakes into Ontario. Models continue to advertise one more
southern stream shortwave lifting north-northeast along a cold front pushing
east through southern Michigan. There may be a bit of a lull in the precipitation
early Monday as we look to fall in a region of subsidence between
the two shortwaves. There remains lower confidence in timing of any
of these features as models have been continually slowing down due
to not handling the deepening of the system effectively yet. So will
start off the day with chance probability of precipitation and increase later in the
afternoon when the wave looks to lift through. The timing is
presenting additional problems regarding instability ahead of the
cold front. Currently models are showing the warm sector of the wave
just skirting to the south of southern Michigan...yet they keep some hint
of instability along the front in Southeast Michigan. If the system can slow down
a bit more allowing the center of the wave to lift west of the
Detroit metropolitan...we may get a little boost in instability from the
warm sector clipping the area. Moral of the story is that will leave
the mention of thunder in the forecast Monday afternoon.
Expectations are for rain showers along the front with a few rumbles
of thunder possible.

The real push of cold air comes with a cold front positioned along
the middle level trough axis pivoting through early Monday night. The
northern stream of the jet will be aligned north to south which will
funnel cold air into the region. With the upper low stalling over
the area into the weekend...this cold air advection pattern will remain in place
through the end of the week. 850mb temperatures will fall below 0c Monday
night...dropping to near -10c by Thursday night...before ridging
looks to displace the low eastward over the weekend. The airmass
will be much drier with precipitable waters around 0.3 inches which will result in
only some chance or slight chance probability of precipitation as additional lobes of
vorticity rotate down around the low through the region through middle
week. Otherwise look for high temperatures to struggle to hit 50 at times
with lows falling to around freezing.


High pressure centered over the Ontario/Quebec border has set up
easterly flow across the region. This flow will increase today as
low pressure deepens and lifts toward the Great Lakes from the
south. Small craft advisories have been issued for all nearshore
zones as gusty winds build wave heights along the shorelines now
into early Monday. Though some gusts may approach gale force...the
marine layer looks to be fairly stable which will limit this
potential. After stalling a bit over the region tonight...the low
will lift northward to near James Bay by Monday night. This will
allow winds to gradually relax through the day Monday.


DTX watches/warnings/advisories...
Lake Huron...Small Craft Advisory from 4 PM this afternoon to 2 PM EDT Monday
for lhz441>443.

Small Craft Advisory from 10 PM this evening to 2 PM EDT Monday for

Lake St Clair...Small Craft Advisory until 8 am EDT Monday for lcz460.

Michigan waters of Lake Erie...Small Craft Advisory until 4 am EDT Monday for lez444.



short term...jvc
long term....drk

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