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Area Forecast Discussion (AFD)

Issued by Gaylord, MI (APX)

FXUS63 KAPX 201800

Area Forecast Discussion
National Weather Service Gaylord MI
200 PM EDT Fri Jul 20 2018

Issued at 1134 AM EDT Fri Jul 20 2018

Morning composite analysis reveals stacked low pressure over
south-central Wisconsin, associated upper jet core dives through 
the base of the trough into the Ohio Valley with a weaker upper
jet piece running up through lower Michigan. Low-mid level
moisture plume wrapping up around the system has been slowly
leaning into lower Michigan this morning, and (aided by a modest
overnight low level jet) swinging a narrow band of showers (and 
some thunder) slowly into lower Michigan. Convection has been 
tending to diminish, but still holding on with some spotty heavier
showers as well as lightning mainly south of M-55. Another batch
of showers appears to be developing up through NE lower Michigan,
along that moisture gradient and possibly aided by some weak-ish
upper jet forcing.

Rest of today and tonight: Stacked low will slowly move through 
southern Lake Michigan through tonight, heading into southern
Lower Michigan for Saturday, with broad but modest QG forcing for
ascent increasing across northern Michigan. Some decent afternoon
instability forecast, to the tune of 500-1500 J/KG in a corridor
spanning northern lower Michigan, particularly the western part of
the state. So, we should see an expansion of showers/thunderstorms
as we go through the afternoon and likely persisting through 
tonight as the stacked low and strongest synoptic forcing moves 
into lower Michigan.

Severe weather threat: Strongest flow aloft and resulting low-mid
level shear is this morning, weakening through the afternoon to 20
knots or less as the instability increases, not so conducive to
well organized updrafts. But, there is decent instability and some
decent downdraft CAPE pushing 800 J/KG, and freezing levels 
around 12K feet. Combined, I could see a few "pulsey" storms 
producing marginally severe wind gusts and hail in line with the 
current SPC Day One. 

UPDATE Issued at 333 AM EDT Fri Jul 20 2018

Latest Day 1 outlook brings a marginal risk for severe storms for
large hail and damaging winds to nrn lower. Still think the
instability may be hard to acquire, especially the GTV Bay region
where clouds are thickening up and diurnal heating will be
suppressed. We will start out with some good heating elsewhere,
but bulk shear is quite low for organized storms (see below) and
pulse type still expected. DCAPE is fairly impressive of around 
800j/kg, so maybe a severe downdraft in a stronger storm. Not much
change in thought on large hail and getting a strong enough
updraft to support large hail, but do think it'll be "in the
neighborhood" for lower end threshold 1" hail in a stronger
storm. Have updated the appropriate products.


.NEAR TERM...(Tonight)
Issued at 139 AM EDT Fri Jul 20 2018

High impact weather potential: Chances for non-severe thunderstorms 
at times into tonight, mainly nrn lower Michigan.

Pattern synopsis and forecast:

The moistening process is underway after sitting in a dry air mass 
for a few days. A closed off upper trough and it's associated 
occluded sfc low were spinning around the MN/WI/IA border, within an 
axis of 1.50" to 1.80" PWATS that stretched throughout the 
Mississippi valley. Numerous showers on the backside of the system 
were across much of MN and nrn Wisconsin, while storms entered the 
mix from srn Wisconsin into parts if Iowa and Illinois, within the 
deeper instability and this system's strong forcing. The forcing has 
contributions from strong DPVA and theta-e advection, along with 
some left exit upper jet divergence. The batch of storms working 
across Illinois were within the true warm sector and greatest 
instability. In nrn Michigan, our mostly clear skies were allowing 
for radiational cooling, but increasing low level srly winds off the 
sfc were keeping the BL at least a little mixed, especially higher 
exposed areas. Temperatures were in the upper 50s and 60's. 

The low pressure system will slowly edge it's way east, ultimately 
settling somewhere around srn Lake Michigan by late tonight. The 
deeper moisture will worked into nrn Michigan with PWATS increasing 
from roughly 0.75" to around 1.70" by late today and tonight. The 
strongest aforementioned forcing and warm sector will get shunted 
well south of Michigan, within the steeper lapse rates and greatest 
instability. Nrn Michigan, while in the high PWATs, will only be 
working with pockets of WAA/DPVA from individual vort maxi that 
rotate around the eastern and nrn flank of the low pressure system. 
This will set off scattered to numerous showers that will impact the 
region at times. The best chances for showers and storms will mainly 
be nrn lower Michigan, closer to the low and our deepest moisture 
and strongest forcing. MUCAPES/MLCAPES over several hundred to maybe 
just above 1000j/kg is now suggested, much better than what was seen 
in yesterday's data. Despite this better expected instability, 
severe storms are not expected. Doubt the instability can result in 
strong enough updrafts to support large hail, especially with 
freezing levels up around 12.5kft. Pulse type storms anticipated as 
well, as storm organization will be limited due to 0-6km bulk shear 
at generally 20kts or less. 

Highs may be a little tricky, generally in the lower half of the 
80s, but extensive thickening cloud cover may result in lower 
readings, especially in and around the GTV Bay region where the 
deepest moisture/clouds will be moving in this morning. Lows tonight 
rather mild under continued solid cloud cover, in the mid and upper 
60s, possibly not getting out of the lower 70s in a few locales.


.SHORT TERM...(Saturday through Sunday)
Issued at 139 AM EDT Fri Jul 20 2018

...Showers Saturday give way to a mostly dry Sunday...

High Impact Weather Potential: Slight chance of thunderstorms 

Pattern Forecast: By the start of the forecast period Saturday 
morning, vertically stacked low pressure is expected to be situated 
across southwestern MI/southern Lake Michigan with several lobes of 
mid-level energy pinwheeling across the state. Low pressure will 
very slowly meander toward the northern Ohio Valley by early Sunday 
morning before becoming washed out across the southeastern states 
early next week.

Primary Forecast Concerns/Challenges: Precip chances throughout the 
upcoming weekend.

Plenty of deep layer moisture expected to be overhead by Saturday 
morning evident by the progged 1.70-1.75 inch PWATs. Shower activity 
is expected to be common across much of the forecast area throughout 
the day Saturday, most numerous across northern lower. Despite most 
guidance still kicking out anywhere from 250-500 J/kg of MLCAPE, 
still not overly enthusiastic with thunder chances through the day 
given extensive cloud cover limiting diurnally driven instability, 
along with poor lapse rates. Certainly any embedded thunderstorms 
that are able to develop could produce locally heavy rain (K-Index 
~35) with such a moisture-rich air mass in place. Rain totals during 
the day Saturday expected to vary from close to half an inch 
along/south of M-72...tapering to 0.20 inches across the Tip of the 
Mitt with even less anticipated across eastern upper. Do have some 
concerns that an east/east-northeast wind out Canada could allow for 
more dry air entrainment across northern Michigan than currently 
modeled. Have refrained from trending the forecast in this direction 
at this point, but if this were to come to fruition, shower coverage 
and inherent QPF amounts would certainly need to be revised. Given 
extensive cloudiness...Saturday's high temperatures expected to 
range through the 70s area-wide.

Shower coverage diminishes Saturday evening as the best forcing and 
moisture begin their southward shift. Still anticipating lots of 
cloud cover throughout the night into Sunday morning given plenty of 
lingering wrap around mid-level moisture with only peeks of sun 
arriving for Sunday afternoon. High temperatures climb a few 
degrees...into the low 80s for most.

.LONG TERM...(Sunday night through Thursday)
Issued at 139 AM EDT Fri Jul 20 2018

High Impact Weather Potential...

High Impact Weather Potential: Minimal.

Another fairly quiet stretch of weather beyond this weekend as the 
only threats for precip arrive Tuesday and again Thursday, both tied 
to weak and moisture-starved cold fronts expected to produce nothing 
more than isolated to scattered showers at this juncture. Warm high 
temperatures Monday-Tuesday...in the mid-upper 80s for many before 
cooling back toward late July normals (77-81 F) post frontal passage 
for the midweek timeframe.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 200 PM EDT Fri Jul 20 2018

...Increasing chances for thunderstorms...

Stacked low pressure system is over south-central Wisconsin and
will slowly move across Lake Michigan into southern lower Michigan
tonight through Saturday. Still anticipate showers/thunderstorms 
to develop across portions of lower Michigan later this afternoon 
into the evening and potentially impact the terminal sites.
Additional showers and some thunderstorms will continue to rotate
around the system through the day Saturday. Bottom line, unsettled
weather anticipated across northern lower Michigan. 

However, flight conditions will remain mostly VFR this afternoon
through tonight, save for brief reductions within any showers/storms.
Looking upstream, there are lower (MVFR and IFR CIGS across 
Wisconsin on the northern side of the low that may bleed into the
region Saturday morning. Will have to keep an eye on that.


Issued at 139 AM EDT Fri Jul 20 2018

Low pressure will slowly meander down into srn Lake Michigan today 
through tonight, then over into the Ohio valley by Sunday. Deep 
moisture overtakes the Great Lakes through the day and tonight and 
lingers through Saturday, with pockets of showers with storms at 
times. There is an outside chance (low confidence) that a strong 
storm will be able to produce damaging winds and large hail this
afternoon into early this evening. Things dry out through Sunday 
as the system heads south. The gradient does tighten up enough for
some advisory level winds through the Straits and for the NE 
lower nearshores, despite neutral to slightly stable conditions 
over the Lakes. This will mainly be for today and tonight, but 
very possibly into Saturday as well. 


LH...SMALL CRAFT ADVISORY until 7 AM EDT Saturday for LHZ349.
     SMALL CRAFT ADVISORY until 5 PM EDT Saturday for LHZ345>348.
LM...SMALL CRAFT ADVISORY until 7 AM EDT Saturday for LMZ341.
     SMALL CRAFT ADVISORY until 5 PM EDT Saturday for LMZ342.