Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
352 am EDT Monday may 4 2015
Short term... today and tonight
After an outstanding weekend of dry and warm weather...an active
precipitation pattern will get underway to start the week. Beginning
this morning...showers and a few thunderstorms will mark the leading
edge of increased low level moisture advancing through lower
Michigan. Morning coverage will be greatest from Flint to the Tri
Cities and northern thumb but all areas will have at least a chance
of showers as the moisture surge progresses through the region. The
cold front approaching from the west will then take over during the
afternoon. That is when the highest concentration of showers and
storms will shift generally south of the Interstate 69 corridor
through early evening. Afternoon thunderstorm potential will be
strongly dependent on recovery of surface based instability that will
be limited by morning clouds and precipitation.
Radar observations indicate showers and a few thunderstorms
expanding in coverage over SW lower Michigan since midnight. This is
occurring on the leading edge of the low level moisture axis with a
modestly increasing low level jet...both of which are generating a
typical pattern of nocturnal elevated instability. The 00z models
have a reasonable handle on this scenario judging by Theta-E and
wind fields in the 850-700 mb layer. Prefer those that are more
bullish on the maintenance of morning quantitative precipitation forecast over Southeast Michigan now that
moisture advection is proving adequate to generate convection. For
now...plan to nudge morning probability of precipitation up into the categorical range in
the Tri Cities and into high likely southward along the U.S. 23
corridor to account for southward expansion along the moisture
There is good model agreement indicating the cold front is on pace
to be through the Tri Cities and northern thumb by early afternoon.
Expect it will take until then for boundary layer temperature
recovery to occur on the heels of the morning showers and convective
cloud debris. This suggests leaning the forecast toward the lower
end of the model range for surface based cape. The NAM continues to
advertise cape reaching 1500-1800 j/kg subject to surface dewpoint
rising into the lower 60s as surface temperature recovers into the
middle to upper 70s. The rap and GFS also indicate surface dewpoint
reaching the lower 60s but produce cape closer to 1000 j/kg with
main difference being shallower middle level lapse rates. This is
generally preferred considering the warm large scale upper air
pattern...especially for this time of year. The one wild card to
monitor is the hint of mesoscale convective vortex development over the Midwest that could
help a few storms overachieve. With that in mind...the Storm Prediction Center outlook
for marginal looks reasonable before the front settles south of the
Ohio border by midnight.
Long term...Tuesday through Sunday
Middle level westerlies take hold Tuesday and Tuesday night along the
northern periphery of slowly expanding southeast Continental U.S. Upper
ridging. This positioning will effectively anchor the surface
reflection of the exiting cold frontal boundary in vicinity of of the Ohio/Michigan
border during this time. Continued convergence across the frontal
slope augmented by a combination of both low amplitude impulses
and/or remnant mesoscale convective vortex activity and some renewed isentropic lift will
support at least a chance for shower development. This pattern
providing the usual challenges in defining both possible coverage
and timing...some dependence on how nocturnal convection expected to
Blossom over the Midwest tonight eventually spills downstream.
Greater potential will lie toward the south given the frontal
positioning and less influence from drier air nosing in from the
north. Ribbon of highest Theta-E will reside along and south of the
boundary...but overall moisture quality certainly remains adequate
/pw holding at 1.3"/. Low level anticyclonic flow filling across
northern lower Michigan will sustain a more stable low level easterly flow
locally. This will keep instability at a minimum...confined above a
firm inversion now in place up through 900 mb. This easterly flow
combined with a higher degree of cloud cover and prospects for
showers to disrupt the heating cycle will cap highs in the lower
and middle 60s on Tuesday.
Upper ridge axis will move across Southeast Michigan Wednesday and
Wednesday night. A slower low level response will likely retain a
longer residence time of east/southeast flow locally...leaving the frontal
slope draped over or just to the southwest for the bulk of the
daytime period. 00z model guidance holding with just very limited
precipitation potential owing to both the degree of stability/upper
ridge positioning and lack of shortwave energy/remnant convective
debris to provide an additional focus. Certainly plausible a few
showers emerge as weak ascent persists along the elevated frontal
slab /900-700 mb/...but confidence remains on the lower end and will
maintain a limited mention at this time. The upward adjustment in
heights and greater prospects for some peaks of sun will nudge highs
up a category or two relative to Tuesday. Highs upper 60s to lower
A period of deeper southwest flow for the latter half of the
week...dictated by high amplitude troughing across the intermountain
west and upper ridging edging toward the East Coast. This will
provide a more noteworthy increase in the temperatures for both
Thursday and Friday. Highs making a run toward the lower 80s by
Friday. Warm/dry profile with negligible forcing will minimize
convective chances on Thursday. Similar environment on Friday per
00z GFS but European model (ecmwf) provides a progressive solution running a lead
wave over the ridge and dragging the attendant cold front into the
region by peak heating. Lower end chance probability of precipitation remain appropriate
to highlight the potential and uncertainty for the next round of
showers/thunderstorms during this time window.
Potential for thunderstorms across all marine areas through tonight
as a cold front moves across the region. Southwest winds today will
shift to northwesterly by evening. Light easterly winds will then
develop for Tuesday and Wednesday as high pressure builds across the
northern Great Lakes.
Aviation...issued 1159 PM EDT sun may 3 2015
Widely scattered showers will work into kmbs/kfnt late tonight into
Monday morning in advance of approaching shortwave and cold front.
This activity will likely fade in the diurnal convective minimum
portion of the day as it outruns any upper support. Additional rain showers
and perhaps a few thunderstorms and rain showers will redevelop by midday into late afternoon
and early evening as the cold front itself progresses into the area.
This will occur first kmbs/kfnt and then shift southward through the
rest of the area 20z-22z and beyond until the cold front clears the
taf sites in the evening and precipitation chances wane.
For dtw...cold front will move into area late in the forecast after
00z Monday...but prefrontal trough will bring scattered rain showers to the taf
site by afternoon. While a thunderstorms and rain or two are possible...confidence
remains low given weak/modest instability. VFR ceilings will give way to
occasional MVFR with this activity...but return to VFR after 02z as
frontal passage occurs and the front then sinks south of the area.
//Dtw threshold probabilities...
* medium for ceilings at or below 5000ft Monday afternoon and evening.
* Low for thunderstorms and rain showers Monday afternoon.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
You can obtain your latest National Weather Service forecasts online
at www.Weather.Gov/Detroit (all lower case).