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Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
530 am EDT Sat may 27 2017



Aviation...

Any br or low clouds this morning will dissipate quickly after
sunrise. Diurnal heating will then allow a diurnal cu field to
develop this afternoon as a shallow layer of moisture remains under
an inversion. There is some uncertainty as to whether there is
enough moisture for a broken coverage, however. Bases are expected to
remain below 5000 feet. Winds today will remain light out of the
east, generally under 6 knots.

//Dtw threshold probabilities...

* low for cigs aob 5kft from 15z-01z Saturday.

&&

Hydrology...

Showers and thunderstorms are likely Sunday, mainly during the
afternoon and evening as low pressure lifts across the area. Basin
average rainfall amounts are forecast to range between one-quarter
and one-half inch, but precipitation totals will vary widely from
location to location due to the hit and miss nature of showers and
thunderstorms. Storms could be on the strong to severe side,
producing very heavy rainfall which could cause ponding of water on
roads and in poor drainage areas.

&&

Previous discussion...
issued at 418 am EDT Sat may 27 2017

Discussion...

Holiday weekend - forecast package highlights.

1. Outstanding weather today. A good amount of sunshine with
temperatures warming into the middle to upper 70s. Cooler near the
Lake Huron shoreline.

2. Increasing confidence in numerous to widespread thunderstorm
activity on Sunday. The potential will exist for large hail and
damaging wind gusts. The potential will also exist for tornado
development. Uncertainty in timing remains, however, latest trends
have supported greatest potential for the thunderstorms will be in
the 5 PM to 11 PM ldt window.

Confidence is also increasing in quiet, very little precipitation
activity Sunday morning through the noon hour.

3. Cooler conditions, in the lower 70s Monday. Scattered showers
with some thunderstorms developing during the afternoon,
particularly north of the M 59 and I 69 corridors. No severe weather
risk Monday.

Today. The northern edge of the persistent mesoscale convective system has been evaporating
while quickly stripping out of southeastern Michigan. Could see a
sprinkle or two before daybreak, otherwise, the precipitation has
largely ended. Model data continues to suggest enough near surface
moisture exists to support some fog, particularly in The Thumb down
through the central part of the cwa. Haven't really seen much in
surface observations to this point, but decided to include a patchy
fog mention through daybreak.

Really a good anticyclone signal today throughout the depth of the
troposphere. Models show a dominant shortwave ridge at 300mb with
good continuity for a compact and localized anticyclonic vorticity
bubble tracking right through the cwa. Additionally, there is a
decent lake aggregate ridge response. Satellite trends this morning
do nothing to dispel the idea that a good amount of sunshine will be
observed. Undoubtedly, there will be a cumulus response, but models
soundings are lean on lapse rates and the look is more of a flat
convective depth. This should set up a great day with temperatures
for a majority of the area rising into the middle to upper 70s.
Warmest temperatures appears set for the Tri Cities region.

Sunday. An interesting setup. Models are largely in agreement that a
number of features at the synoptic scale will come together over
southeastern Michigan to support a severe weather risk. The most
dominant of the features will be the larger wavelength, Canadian pv
anomaly that will dig into the far western Great Lakes basin. While
this is occurring, the southern branch to the upper level jet is
shown to merge with the pv anomaly. The combination of left exit
region dynamics from the speed maximum aloft with the apex of a very
well developed low to midlevel Theta E ridge will result in a triple
point or surface low center lifting northeastward though lower
Michigan. Forecast timing has the surface low by the Tri Cities by
00z Monday.

A warm front will reside immediately along the forward flank of the
Theta E ridge, connected to the surface low/triple point. The warm
front is forecasted to lift northward from the Detroit Metro at
approximately 20z up into The Thumb by 00z. Given the limitations of
nwp, it is always difficult to say with absolute certainty what
surface dewpoints will be in an advection event. Looking at plan
view data along with forecast soundings, a safe expectation is for
surface dewpoints to eclipse the 60 degree mark and into the low 60s
during the afternoon. Without question the big negative for this
setup will be poor midlevel lapse rates. In fact, there is a pretty
formidable subsidence inversion progged early in the 5.0 to 10.0 kft
agl layer. Thinking at this time is that 1000 j/kg of MLCAPE will
develop with perhaps as much as 1500 j/kg possible if localized
pooling of moisture can occur along the warm front. In addition to
the instability, data suggests a good amount of shear will exist in
the warm frontal region with 35 to 40 knots of 0-6 km bulk shear and
25 knots of 0-0.5km bulk shear. Therefore, it is expected that
supercells are the most likely of storm Mode. A frontal wave
response related to the midlevel Theta E ridge lifting into lower
Michigan will support the development of showers and thunderstorms.
The potential will exist for severe weather across all of
southeastern Michigan. The best of the potential will exist along
and immediately on the cool side of the warm front. The severe
weather threats will be damaging wind gusts to 60 mph with mature
storms and downdraft development, some large hail to 1 inch. Backed
southeasterly surface flow ahead of the warm front which will result
in near surface directional shear, resulting in the potential for
tornado development.

The failure mechanism for severe weather development Sunday will be
if a combination of clouds and poor midlevel lapse rates will
significantly limit instability. Given the combination of late
afternoon timing, a climatologically favored period in late may, and
directional shear along a warm front, the decision was made to err
more aggressive on messaging for a potential severe weather event.

Monday...the upper level low will push down directly into the
central Great Lakes. A double barrel, merged, upper level jet
configuration should act in negating the southern push of the cold
front. With some breaks in clouds across the southern forecast area
temperatures will again climb above 70 degrees. However, the most
noticeable aspect of the forecast will be much cooler dewpoints.
Clouds will also push down across all of the area during the
afternoon. Some question about who exactly will see scattered
showers with possible thunderstorms during the afternoon. The nwp
data wants to limit the precipitation chances to the north of M59
and i69 corridors. However, not into splitting hairs at the moment
and want to keep a chance in for all areas. No severe weather risk
Monday.

Marine...

Fairly light winds are expected over the region today as high
pressure builds back over the Great Lakes. Winds will stay
moderately light Sunday as low pressure lifts through the area, but
showers and thunderstorms will become likely across all of the
central Great Lakes. Some of these storms may become severe. Winds
will turns southwesterly behind a cold front by late Sunday evening,
then strengthen Monday through Tuesday as low pressure stalls over
Ontario. Small craft advisories may be needed, especially over
Saginaw Bay.

&&

DTX watches/warnings/advisories...
Michigan...none.
Lake Huron...none.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
&&

$$

Aviation.....Hlo
discussion...cumulonimbus
marine.......hlo

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