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Area forecast discussion 
National Weather Service Gaylord Michigan 
355 PM EDT Friday Jul 18 2008 


Discussion...surface map this afternoon very chaotic...with 
broad/weak low pressure located north of lakes Superior/Huron. A 
surface trough extending southward from the low (detectable earlier today) 
is now being masked by lake breezes and lake-water pressure 
differences. Some scattered convection has fired over NE lower...in 
an area of low level convergence and laps-derived sbcapes around 
1500 j/kg. Though deep layer shear is decent in this area (near 30 
kts)...expect high freezing levels to preclude any severe weather 
early this evening...though locally heavy rainfall again possible. 


In addition to the ongoing convection...additional scattered 
convection will develop across parts of northern lower Michigan late this 
afternoon and early this evening. Activity is expected to push east-southeast 
and diminish...as aforementioned weak front takes shape by late this 
evening -- likely extending from Southern Lake Huron across central lower 
Michigan to southern WI by 06z. Convection should end by 02z. Then...with light 
winds and lack of a strong push of drier air this afternoon... 
expect fog and possibly low clouds to develop around/after midnight 
area-wide. 


Have also retained a small chance for convection late tonight across 
far southern areas...just north of front. GFS -- in particular -- suggests 
that modest 800 mb low level jet and a weak 500 mb short wave may trigger some 
nocturnal convection over southern WI later tonight within an area of 800 mb 
dew points near +13c. If convection forms...expect it to drift east 
into central/Southern Lower Michigan late tonight. 


Otherwise...partly to mostly cloudy tonight with lows in the middle 50s 
north to 60-65 south. 


Smith 


Saturday...somewhat determined by convective outflow...stalled 
boundary will remain stalled roughly from around mbl to taw the 
entire day. No real large-scale forcing mechanisms to speak 
off...outside of the surface boundary. May have to watch remnant 
mesoscale convective vortex over South Dakota as it is timed to arrive right around 12z and 
will clear the area by around midday. Models appear to point 
towards low level jet forcing developing ahead of the wave across southern 
WI/northern Illinois/Southern Lake Michigan late tonight which would then 
move across central and Southern Lower Michigan tomorrow morning. 
Hard to say how far north this mesoscale convective system will reach and how much 
interaction there will be along the front. With the primary 
moisture plume to the south of the County Warning Area...feel fairly comfortable 
going low chance wording for the morning south of M-72 before 
instability builds in the afternoon. Lower surface dewpoint air 
will try to make inroads across the northern half of the County Warning Area...while 
moisture pooling along the boundary will maintain the sticky airmass 
in place to the south. Light synoptic wind fields will Mean Lake 
breezes will be present during the afternoon with the NAM/GFS 
seeming pointing the most low level convergence over northwest lower. 
Modifying model soundings indicate up to 1000 j/kg of ml cape south 
of M-72 tomorrow afternoon...and with plenty of boundaries 
present...have to maintain a chance of storms from the tip of the 
mitt on south. 0-6km shear reaches up to about 30kts...so not out 
of the question that an isolated storm could produce damaging winds 
and large hail. 


Saturday night through Sunday night...numerical guidance points to a 
Oregon shortwave having reached the western Great Lakes with a weak 
surface reflection along the stalled 850mb front...which stretches 
from central Minnesota to central lower Michigan...Saturday 
evening. Operational model solutions are not in good agreement with 
how this system will evolve so will attempt to make a blend of the 
most likely scenario. Location of the front will be impacted by any 
previous convection today through Saturday afternoon. Assuming 
frontal positions go somewhat as planned...this surface low will track 
along the boundary towards the Green Bay region by Sunday morning. 
Several factors seem to come together to suggest a potential mesoscale convective system 
developing over Minnesota/WI and arriving late Saturday 
overnight...including middle/upper level support in the form of 
divergence and 700-500mb -divq...along with a 30+kt low level jet nearly 
perpendicular to the 850mb thetae gradient. Any mesoscale convective system will then track 
across central lower late Saturday night into Sunday morning with 
subsidence arriving behind the middle-level wave and the front...as it 
gets pushed south Sunday afternoon into Sunday night. With all this 
being said...plenty of room for error here as it appears 12z NAM/GFS 
are contaminated somewhat by convective feedback. Have instead 
relied on the 00z European model (ecmwf) which has been showing good continuity over 
the past couple days. If the 850mb gradient does lift far enough 
north for northern Michigan to lie in the instability 
sector...decent 0-6km shear would suggest some severe thunderstorm 
chances...although primary threat will most likely be heavy rain 
with precipitable waters  approaching two inches and ample forcing present. 


Monday through Friday...medium range models appear in decent 
agreement with the upper air pattern through next week. These 
models continue to advertise the shortwave trough...which passes 
across the western Great Lakes Sunday...will join forces with 
residual troughing across Quebec to form an anomalous closed upper 
low over the eastern great through midweek before lifting 
northeast. With northern Michigan on the subsidence side of this 
upper low...dry expectations remain reasonable for much of the week 
as a result. Primary shortwave is prognosticated to have cleared the area 
by 12z Monday...but additional energy upstream combined with upper 
trough overhead may generate a few diurnal showers on 
Monday...although chances are small. Surface ridge axis then builds 
in across the central Great Lakes Tuesday through Thursday. Will 
reintroduce precipitation chances on Friday as return flow/warm air advection kicks back 
in. Temperatures will be seasonable if not below average Monday-Thursday 
with some warming heading into Friday. 


Mpc 


&& 


Aviation...issued 159 PM EDT Friday Jul 18 2008 


Areas of MVFR SC/cumulus persist is some locales. However...with 
additional daytime heating these clouds should gradually lift into a 
VFR cumulus deck. Daytime heating and approaching weak 
front...however...will also result in spotty shower and thunderstorm 
development. This activity should diminish this evening as weak 
surface front/trough sets up just S of region. Shallow moisture 
behind front is expected to result in low cloud and fog 
development...with IFR conditions possible at tvc/pln/apn after 
about 04z. 


Smith 


&& 


Apx watches/warnings/advisories... 
Michigan...none. 
Lm...none. 
LH...none. 
Ls...none. 
&& 


$$ 

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