National Weather Service United States Department of Commerce

Forecast commentary -

Holiday weekend - forecast package highlights.

1. Firmed up timing for pops today. A dry morning period with pops ramping up post 18z. Shower and
thunderstorm event ( numerous pops ) expected from southwest to northeast between 21-03z.

2. Isolated to scattered severe weather possible between 21-03z for all of southeastern Michigan.
Focus for the severe weather threat will be along and ahead of a warm front lifting through the
area. Severe weather threats include: damaging wind gusts to 60 mph, large hail to 1 inch, and a
low potential for tornado development.

3. On Monday, temperatures will reach the lower to middle 70s areawide but will feel cooler because
of falling dewpoints. Warmest temperatures south and east of glacial terrain within the Detroit
urban heat island. Cooler near the Lake Huron shoreline.

4. Isolated rain shower or thunderstorm chance during the very late afternoon or evening hours
Monday, particularly north of the M-59 corridor. No severe weather potential Monday.

Quiet weather is forecasted this morning. The slab of high stability in the 6 to 12 kft agl layer (
see 28.00z KDTX raob ) is progged to hang around into the afternoon while lowering a couple of
thousand feet. The back edge of the upper level ridge needs to exit the region. Will likely see some
echoes show up on radar, but expecting this to remain virga through 18z.

A significant lobe to the Canadian upper level PV anomaly will work through the base of the trough
this morning and continue to a negative tilt over Wisconsin this afternoon. This action along with
left exist region dynamics overspreading the region will result in a broad area of 1000-500mb
geopotential height falls over the central Great Lakes. This organized forcing will be strong enough
to draw richer low to midlevel theta-e content from parts of far southern il/in northward into
southeastern Michigan. No question, the best thing going for this setup will be the distinct, well
developed low to midlevel theta-e ridge that will serve as the focal point in the frontal wave that
will roll across the state.

Not a whole lot has really changed in the latest iterations of the NWP or from previous forecast
thinking. The later timing that has been bandied about for the past couple of days has held on. The
dominant forcing that is expected to govern shower and thunderstorm development today will be the
low to midlevel warm advection that will occur along and ahead of a warm front. A slight shift in
the 00z suite was for a few solutions, most notably the operational NAM, resolving a slightly weaker
surface low reflection developing at the triple point along with a flatter release of the low.
Really difficult to say definitely if this slightly southern solution will end up verifying. The
reason is that dynamical forcing today is not high end or extremely crisp. Looking through the
gradients of both the mass and kinematic fields, the feeling is the SPC swody1 graphic may be overly
reliant on the NAM. Therefore, the belief is that potential for isolated to scattered severe
thunderstorms exists for all of southeastern Michigan including the Tri Cities and the tip of the
Thumb. The biggest factor in this decision is the 700-500mb warm front will lift northward through
the cwa.

The largest negative factor for severe weather development today will be very poor midlevel lapse
rates. Will likely require solid lower 60 dewpoints at more than skin level, in addition to, some
peaks at sun or higher transparency high cloud to result in mlcapes of 1000 j/kg. This is certainly
doable given upstream dewpoint observations in southern il/in. Cannot say for certain though with
the typical NAM already overdoing dewpoints locally this past evening and MOS guidance some 3 to 5
degrees lower with dewpoints than prior predictions. Now for the shear. Really kind of split as deep
layer shear numbers have improved regionwide with widespread 40 knots accessible, while in contrast,
near surface shear both from a directional turning perspective and 0-0.5Km shear is notably less.
With that said, the heavy component of a flatter and weaker surface low is playing into that. At
this juncture, do not want to move much from prior thinking. Bottomline, deep layer shear will be
more than adequate for mesocylone development. This supports an isolated to scattered severe threat
today for damaging wind gusts to 60mph, large hail to 1 inch, along with a low potential for tornado
development. Peak of the event is still expected 20-03z for all of southeastern Michigan along and
ahead of a warm front.

Weather and skies are expected to clear out rapidly overnight in the wake of the event as very
strong signal exists for 925-700mb dry slot overspreading the area.

The main upper level low will be slow to dig into southeastern Michigan Monday. The most obvious
thing going on Monday is an unfavorable coupling of upper level jet streaks over Michigan and Ohio.
Cyclonic vorticity advection will eventually arrive but it will be later in the day. Lapse rates and
depth of moisture are shown to be weaker or less favorable for precipitation than earlier progged.
Isolated or scattered pops appears more than adequate.

. . posted at 412 AM EDT Sun May 28 2017