Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 201957
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
357 PM EDT Fri Oct 20 2017

.DISCUSSION...

Near Term / Through 6 AM Saturday morning

As of 355 PM EDT...A tranquil and unseasonably warm weather day
continues across southeast Michigan as surface high pressure remains
anchored across the upper Ohio River Valley. Aloft, upper-level
ridging is in place. Generally light south to southwest flow
continues to prevail with the return flow around the high. This
return flow has allowed for a surge in midlevel dry air across the
region, which has resulted in dry weather and sunny skies.
Temperatures have warmed well above normal for this time of year,
with temperatures warming well into the 70s for most areas, and even
upper 70s near US 23.

Upstream, an increase in high cloud cover was noted across the
Mississippi River Valley in association with a deepening longwave
trough which will impact the region during the short term period
(more in the short term section of the discussion). This cloud cover
will begin streaming into the area tonight, and combined with a
lingering southerly wind due to a tightening pressure gradient, will
help keep low temperatures elevated in the low to mid 50s. Sheltered
areas that are able to briefly decouple will be able to see lows dip
into the upper 40s.

Short Term / 6 AM Saturday morning through Monday

Complicated forecast scenario evolves throughout the short term
period, as both the northern and southern stream jet become quite
active and resemble a more typical transition season energetics
pattern. During this period, potent northern stream energy will dive
southeastward from the Pacific Northwest, noted by a strong PV
anomaly dipping as far south as the Four Corners region. This PV
anomaly will help to dig a deep longwave trough across the central
CONUS, but much of the corresponding northern stream jet dynamics
look to be displaced further north as the initial PV surge becomes
pinched off from the larger scale flow.

Meanwhile, a surge in the southern stream subtropical jet looks to
round the base of the longwave trough, with the 12z guidance now
coming into better agreement with a cutoff low developing across the
lower Plains out of the remnant northern stream PV lobe, while the
northern stream jet remains displaced to the north, and a split-flow
regime setting up for the weekend and into early Monday.

Initially in this split flow, it looks to be one last dry day on
Saturday with gradually increasing cloud cover and temperatures once
again reaching well into the 70s with lows Saturday night in the
50s. Then for the latter half of the weekend, a cold front will
cross the region late Sunday into Sunday night, with the trend in
the past few runs continuing to indicate a slowing in the timing of
the front. This front will bring scattered to numerous showers
riding a surge of deeper moisture, with rainfall amounts generally
on the order of one quarter to three quarters of an inch of rain.
Temperatures on Sunday look to spike again into the 70s despite
thickening cloud cover and increasing chances for rain from west to
east late in the day.

Long Term / Monday night through Thursday

Forecast uncertainty continues into the extended period as long
range guidance continues to struggle handling the evolution of the
split-flow regime. While there continues to remain uncertainty in
the forecast, there are increasing trends that a strong low pressure
system (most long range guidance deepens it to sub-990 hPa by the
midweek period!) will develop in the vicinity of southeast Michigan
for early next week. This low pressure system looks to develop in
response to another surge in northern stream PV energy diving south
out of the northern Plains, and interacting with the ejecting upper-
level closed low in an active southern stream. Because of the split-
flow regime leading up to this event (long range guidance tends to
struggle handling this regime beyond 3 days), the degree to which
phasing and amplification of the synoptic pattern occurs is still in
question, and should hopefully be resolved in a clearer picture in
the next 24-36 hours.

Will go with a blended approach of the 12z GFS/ECMWF and various
ensemble members during this timeframe, with the forecast depicting
likely showers Monday night through Tuesday night. Monday night into
Tuesday, there are signals for a potential heavy rain/high wind
event ultimately depending on the evolution of the low pressure
system, and this trend will need to be monitored closely over the
coming days. A drying trend in the wake of the system will be
possible to end the long term period, but again will hinge on the
evolution of the low pressure system.

Temperature-wise for the long term period, a trend back towards
normal will occur, as much colder air is ushered in within the
strong northern stream flow. Daytime highs will return to the upper
40s to upper 50s, with lows in the upper 30s to mid 40s.

&&

.MARINE...

High pressure continues to dominate the weather pattern with
southerly winds 10 to 20 knots through tonight. A trough moving
across the Rockies and through the Plains will push a cold front
through the Great Lakes late Sunday night.  Wind will pick up to
around 20-25 kts out of the south ahead of this cold front as
surface pressure gradient tightens. Showers will be possible Sunday
night through the early part of next week as the next low pressure
system impacts the region. Some gusts could reach around 30 knots by
Tuesday.

&&

.HYDROLOGY...

A cold front and upper trough approaching the region at the end of
the weekend will bring a chance of increased rainfall. Rain chances
begin Sunday night and continue through the early part of next week
as the next low pressure system arrives. There is the potential for
several inches of rain across the region beginning late Sunday and
continuing well into next week. Rainfall amounts will be fine tuned
as the event draws nearer as there remains much uncertainty in the
forecast beyond Sunday.

&&

.PREV DISCUSSION...
Issued at 151 PM EDT Fri Oct 20 2017

AVIATION...

VFR conditions will prevail for this TAF period under south winds
around 5-10 knots. Winds will begin to decrease into the evening.
Few clouds except for occasional FEW-SCT high clouds around 25kft.

//DTW THRESHOLD PROBABILITIES...

* None.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

DISCUSSION...IRL
MARINE.......AA
HYDROLOGY...AA/IRL
AVIATION.....AA

You can obtain your latest National Weather Service forecasts online
at www.weather.gov/detroit.



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