Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 220827 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Detroit/Pontiac MI
427 AM EDT Fri Sep 22 2017

corrected for time of equinox and updated heat index guidance


The first day of fall (equinox at 402 PM EDT) will arrive with a
convincing mid summer feel of record or near record highs again
this afternoon. Early morning satellite imagery and model analysis
fields indicate the upper level ridge axis centered directly over
lower Michigan where it will remain through the weekend. The 00Z
model package is in good agreement showing 500 mb height near 590
dm through Saturday and even approaching 592 dm during Sunday
while broadening in scale from the Great Lakes to the New England
states and mid Atlantic coast. The corresponding surface high will
also strengthen and broaden over the next few days and provide a
light south wind over the region with an onshore component into
the Thumb and into the Detroit metro area. The cooling effect of
the onshore flow will be limited and confined closer to the
shoreline as the lake temperatures are at warm season peak. This
was demonstrated yesterday where Bad Axe reached 90 while Port
Hope only managed 84 for high temps. Interior sections from Flint
to the Tri Cities will remain warmest with another round of highs
in the lower 90s likely as boundary layer mixing reaches closer to
825 mb in that area. The RAP sounding for MBS as an example
offers a mixed layer to 815 mb at 16 C translating to surface
temps of 91-93 F, right in line with readings that occurred
Thursday afternoon. These readings combined with dewpoint near 70
will make heat index in the mid to upper 90s possible for a few
hours during mid afternoon. This is short of heat advisory
criteria (heat index = 100) but warm enough to highlight for the
time of year. Precautions will be mentioned in forecast messaging
for today through the weekend.

The high temperature records for today: 89 MBS, 90 FNT, 91 DTW
all set in 1941.

As warm as surface temperatures will be during the afternoon,
readings above the surface only approach the values considered
warm enough to firmly cap off convection, 20 C/850 mb and 10 C/700
mb. However, a deeper examination of model soundings does show a
more convincing warm layer around 800 mb that will be difficult to
break on just surface heating alone. The building upper level
ridge and broadening high pressure at the surface do not offer any
additional larger scale support to augment peak heating. High
resolution CAM output does show some specks of convection over
interior sections during mid to late afternoon which will be
monitored before adding to the forecast.

After the mid summerlike weekend, expect temperatures to remain well
above normal through Monday and Tuesday before the next front
arrives with cooler air Wednesday into Thursday. Global models time
the system to arrive roughly late Wednesday. There remains about 12
hours of spread among the deterministic solutions likely originating
in the onset of the large scale pattern shift. This transition will
strongly depend on the timing of new height falls over the north
Pacific and associated west coast ridge development. Such a strongly
amplified long wave pattern is usually slower to break down favoring
a later frontal passage in the Great Lakes. The new forecast
database has a chance POP on Wednesday and a strong cooling trend
during the second half of the week. These elements are both good
markers for the change back toward normal late September
temperatures that the front will bring subject only to some timing
adjustments in upcoming forecasts.



Very strong upper level ridge will be situated over the Central
Great Lakes through the weekend, leading to light winds (under 15
knots) and low waves, along with very warm and dry conditions. The
warm and quiet pattern looks to persist into early next week as


Issued at 1153 PM EDT Thu Sep 21 2017


Strong upper ridging will provide dry conditions during the forecast
in terms of rainfall. However, with the long early fall night`s and
light/variable winds, expect patchy fog given the warm, humid
airmass in place with this ridge. Will maintain MVFR to occasional
IFR vsby conditions overnight and introduce for late in the forecast
Friday evening.

For DTW...Fog is expected to form again tonight in the warm, moist
environment. Dense fog is not expected at this time as some degree
of cirrus arcs over area within upper level ridge. Fog will occur
again Friday night as well and may begin to form in the 03z-06z time
frame per various guidance output. Area will be directly under the
ridge Friday night, so calm winds may lead to a better chance at a
period of dense fog.


* None.



Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.




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