National Weather Service - Area Forecast Discussion

FXUS63 KAPX 181519

Area Forecast Discussion
National Weather Service Gaylord MI
1119 AM EDT Tue Sep 18 2018

Issued at 1112 AM EDT Tue Sep 18 2018

The southern edge of the cloud deck is finally beginning to move
north, but the main mass of the low clouds has continued to stream
south. This is beginning to look like a typical northern Michigan
fall day as the cool air streams over the lakes and continues the
low clouds over, at least, N Lower, and portions of C and E Upper.
This makes for a tough temperature forecast as the clouds will
definitely hamper the warmer temperatures expected originally. So
have loaded the NAMNest grids which seem to meet what is going on,
and the expectation that the clouds will be slow to erode. The
temperature curve was close to expectations as well, so have done
some minor adjustments to the NAMNest over the day and have
lowered the temperature expectations.

Confidence in this is medium, as the sun has made some in roads to
eroding the lower level cloud deck in the SW portion of the
forecast area. For the time being, the rest of the cloud mass has
held firm or slightly increased.


.NEAR TERM...(Today through Tonight)
Issued at 332 AM EDT Tue Sep 18 2018

...A much cooler day ahead...

High Impact Weather Potential...None.

Pattern Synopsis/Forecast...The cold front has progressed downstate
early this morning, with high pressure and a cooler, drier airmass
gradually building in from Canada. Dewpoints are falling into the
lower 50s in many spots, about 10 degrees lower than they were at
this time last night. Decaying light showers/sprinkles from over
Lake Michigan earlier have dissipated as they lose support from the
responsible weakening shortwave crossing northern Michigan.

Quiet weather is expected to continue through daybreak and through
the day on Tuesday as high pressure gradually exerts greater
influence over northern Michigan. Isentropically-triggered
convection out over southern Minnesota and northwest Wisconsin at
this early hour is progged to largely dive southeast, so we do not
look to be in its trajectory (despite recent HRRR/RAP runs brushing
our southwest with some showers early this afternoon). As the
surface high slowly drifts across southern Ontario today, we`ll
remain under the influence of weak cold air advection. This should
keep plenty of clouds around through at least this morning, with
some erosion of cloud cover expected from the north by this
afternoon. All in all, a far cry from yesterday`s warmth with highs
today running a good 15 to 20+ degrees cooler, ranging from the mid-
upper 60s north to the low-mid 70s south. Winds will remain
northerly, turning a tad breezy at times this afternoon.

High pressure expected to keep upstream isentropically-induced
convection out of northern Michigan yet again tonight. Partially
clear skies and boundary layer decoupling will promote an efficient
radiational cooling setup, with longer nights also contributing to a
cool night ahead. Current forecast lows range from low-mid 40s north
to low-mid 50s south. If trends happen to favor more clearing, lows
tonight may need to be adjusted even lower.


.SHORT TERM...(Wednesday through Thursday)
Issued at 332 AM EDT Tue Sep 18 2018

High Impact Weather Potential: Chance of thunderstorms Thursday and
Thursday night...perhaps including a few severe storms primarily
capable of producing damaging wind gusts.

Pattern Forecast: Broad ridging is expected to build across the
midsection of the country through the middle of the week with focus
revolving around a well-defined upper level wave and attendant
developing low pressure across the central plains on Wednesday
before trekking to near the northern Ontario/Quebec border by Friday
morning. The system`s warm front is expected to quickly slide
northward across the forecast area Wednesday night with deep
southerly return flow during much of the day Thursday prior to a
cold frontal passage Thursday night.

Primary Forecast Concerns/Challenges: PoPs and associated
thunderstorm potential (severe threat?) Wednesday night through

By the start of the forecast period Wednesday morning, mostly cloudy
skies are expected across much of northern Michigan...gradually
thickening/lowering throughout the day. Wouldn`t be surprised to see
a few isentropically driven showers develop as early as the midday
and afternoon hours, especially across the southwestern portion of
the area, although much better rain chances arrive area-wide after
10 PM Wednesday night continuing into Thursday morning with a
widespread 0.20 - 0.50 inches expected by 12z Thursday.

By Thursday morning, southerly return flow is expected to be
underway across northern Michigan aiding to increase deep layer
moisture...evident by PWs rising from ~1.75 inches to nearly 2.00
inches Thursday afternoon (+2 to 3 SD). While much of the area may
dry out for a time Thursday afternoon/evening (especially across
northern lower), a muggy afternoon is on tap with temperatures
climbing to the mid-upper 70s and dew points creeping back into the
upper 60s for many. Center of low pressure passes to our northwest
Thursday evening with a trailing cold front expected to cross
northern Michigan Thursday night... bringing with it another round
of showers and potentially embedded thunderstorms. Guidance suggests
a meager ~300 J/kg of MLCAPE developing Thursday afternoon, although
uncertainty is rather high with respect to destabilization given
only breaks in the clouds expected. However, even with what looks to
be anemic instability, pre-frontal bulk shear values upwards of 50
kts could potentially lead to a few severe wind gusts in any
thunderstorms, which meshes well with SPC`s latest Day 3 Outlook
that includes all of northern MI in a Marginal Risk and portions of
eastern upper, Tip of the Mitt and northwest lower in a Slight
Risk. At the very least, locally heavy rain will be possible as
any storms should be very efficient rain producers.

Lastly, given the tightening pressure gradient across the Great
Lakes region, winds will become noticeably more gusty, primarily
Thursday afternoon into Thursday night when gusts of 20-30 mph are
expected to be common (highest for coastal communities).

.LONG TERM...(Thursday night through Monday)
Issued at 332 AM EDT Tue Sep 18 2018

High Impact Weather Potential: Minimal.

Perhaps a bit of lingering post-frontal moisture hanging around
during the day Friday and while guidance variability remains high at
this point, it certainly could be cool enough aloft by Friday
afternoon for some lake induced rain showers to develop downwind of
Lake Superior and Lake Michigan (e.g. ECMWF progs H8 temps around +3
C by 18z Friday leading to delta Ts of ~17 C). That scenario doesn`t
look to last all that long given a very dry incoming airmass behind
Thursday night`s frontal passage (H8-H7 RH values falling below
30%). Quiet weather then expected to prevail through the remainder
of the weekend before another system develops across the upper
Mississippi Valley early next work week.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 713 AM EDT Tue Sep 18 2018

MVFR cigs with some pockets of IFR will linger over northern Lower
this morning, affecting all sites but MBL. Otherwise high pressure
over Ontario will slowly build into the area through the day. This
should act to eventually scour out the low cigs towards late
morning or midday with some sunshine for the afternoon. Convection
over Wisconsin this morning will not impact northern Michigan
aside from some debris cirrus. Northerly winds will remain fairly
light today, variable at times especially this afternoon as lake
breezes develop.


Issued at 332 AM EDT Tue Sep 18 2018

High pressure progressively building in from Canada will keep
northerly winds in place today. Will allow the Lake Huron small
craft advisory to expire as scheduled this morning with a
weakening pressure gradient overhead. Winds may become a tad
breezy this afternoon but are expected to stay below advisory
criteria. By Wednesday morning winds will veer to the east.





NWS APX Office Area Forecast Discussion