Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
114 am EDT Friday Jul 25 2014
Conditions will remain VFR with only cirrus increasing overnight
into Friday. A further increase/lowering of ceilings at or above 15kft will
occur late in the forecast as high pressure settles to the south and
a wave of low pressure encroaches on the area. Any shower activity
from this system will hold off until at least after 06z. Light and
variable winds will become southwest...but remain 10 knots or less
for the most part...even during best mixing Friday afternoon.
//Dtw threshold threats...
Previous discussion...issued 328 PM EDT Thursday Jul 24 2014
Short term...through tonight
Quiet night ahead as an area of surface high pressure drift across
southern Michigan. The center of the high looks to position itself right
over Southern Lower Michigan from about 00z-06z Friday. The dry air
advecting in along with increasing subsidence will help scour out
the thin layer of moisture/cumulus this evening bringing clear skies
through the overnight. Winds will also decrease as the veer from
northerly to southwesterly by morning. We will start seeing some
high cloud debris moving into the area late tonight from convection
currently over Minnesota but any low cloud development will hold off until
very close to sunrise as SW flow begins to kick in allowing moisture
to start advecting back into the low levels. The additional dry air
and favorable radiational cooling conditions will allow temperatures
to once again drop to around 50f...with some upper 40s in the
typical cooler locations. Surface dewpoints mixed out into the upper
40s this afternoon which also support these cool min temperatures...near 10
degrees below normal for late July.
Showers and thunderstorms have developed within the deep moisture
plume present over the Great Plains this afternoon. This activity
was initiated along the lead edge of height falls emanating from the
Pacific northwest low and a meridionally oriented 40kt low level jet. The upper flow over
the central U.S. And Great Lakes will will rapidly acquire a
strongly confluent character as eastward progress of said low
dampens the upstream ridge. The resultant transition toward deep
layer westerly flow will force the high Theta-E plume to fold across
the area late Friday into Friday night. Lingering surface high
maintained by middle-level confluence and plenty of dry air will ensure
that upstream activity dries up before it has a chance to affect the
County Warning Area tomorrow afternoon, guaranteeing another dry day Friday. Airmass
modification will add a couple degrees to highs compared to today,
suggesting most places topping out in the upper 70s.
Moisture return getting underway in earnest Friday night through
Saturday as the low migrates into northern Minnesota and comes under
increasing influence of the large northeast Canada vortex. Height
falls will be ongoing through the weekend, helping to gradually
weakening ambient capping with time. The first chance for showers
and thunderstorms will be late Friday night as the previously discussed
Theta-E axis washes across the state and the low-level jet ramps up.
Nothing Worth more than a low chance pop at this time as forcing will not be
terribly impressive and there will be a lot of dry air to overcome.
Severe potential will exist both Saturday afternoon and all day
Sunday. On Saturday, the threat will depend strongly on prior
convective trends and positioning of stationary front, which is most
likely to set up near or just south of the County Warning Area per 00z/12z forecasts.
Upstream convection, energetic westerly flow, and the potential for
morning convection all point to an abundance of clouds, potentially
strong capping, and a grungy/muggy day in general. Destabilization
potential does not look great, but will require monitoring given the
40-50kts of 0-6km bulk shear that will be present.
By Sunday, middle-levels are forecast cool significantly. Meanwhile,
increasing interaction between the northeast Canada vortex and the
approaching upper low will help to whip a corridor of significant
height falls across the Great Lakes during middle/late day. The
stationary front will be forced north while the strongest westerlies
will be shunted southward along the southern periphery of the low
center - although bulk shear values are still prognosticated to remain
adequate - at or above 30kts for most of the County Warning Area. Cool middle-levels
and influx of warmer surface air/dewpoints point to potentially high
cape density and the possibility for a potent severe weather scenario.
Low pressure will continue to be situated over the central Great
Lakes on Sunday night resulting in rains showers and potentially
thunderstorms across the County Warning Area. Strong high pressure will build
quickly into the western Great Lakes by Monday morning and remain
over the Great Lakes through most of the week. This high will bring
mostly dry conditions to Southeast Michigan...however...current
model runs support some scattered showers in the afternoon hours as
diurnal instability develops throughout the daytime hours next week.
High temperatures will remain fairly mild /mid-70s/ through next
Thursday with overnight low temperatures in the middle to upper 50s.
Episodes of thunderstorms will affect the waters Saturday through
Sunday as a warm and moist airmass lifts into the Great Lakes
region. Severe weather will be a possibility both Saturday afternoon
and again during the day on Sunday. Winds...on the other hand...will
remain on the light side through the weekend before ramping up again
early next week as another round of cool air spreads across area
waters. Moderate to fresh north winds with gusts to 25 knots or
slightly higher will be common by Monday.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
You can obtain your latest National Weather Service forecasts online
at www.Weather.Gov/Detroit (all lower case).