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000 
FXUS63 KDTX 231937
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
337 PM EDT Mon Oct 23 2017

.DISCUSSION...

Dynamic mid level circulation and associated deepening surface 
reflection rapidly advancing northeast toward the area this 
afternoon. This process will be accompanied by a substantial deep 
column mass adjustment, as the height fall gradient of a trailing 
mid level wave and accompanying upper jet core interact with the 
inbound system. The subsequent substantial height falls and 
corresponding surface cyclogenesis will leave a deep closed system 
parked over far northern lower MI by tonight. Upstream observational 
trends and recent model guidance continue to depict a surface low 
track roughly from northeast Indiana through Lansing and to near 
Saginaw Bay through the evening. Window of stronger ascent contained 
within the eastern flank of this passing system will provide the 
greatest opportunity for meaningful rainfall most locations going 
forward from mid afternoon through the evening hours. A greater 
convective element noted across the Ohio valley will undercut some 
of the moisture quality, but rainfall amounts into the half inch 
range remain attainable given the underlying forcing. Current 
trajectory does maintain an opportunity for the well-defined mid 
level fgen and deformation axis to pivot into the northern Saginaw 
Valley, with residence time then ultimately dictating rainfall 
amounts. Some contraction in scale of the deepest ascent as upper 
jet coupling briefly drives a greater fgen response could then 
potentially yield a several hour period of heavier rainfall over 
this corridor. Therefore, electing to hold tight with the flood 
watch over Midland and Bay counties until a more definitive signal 
emerges on this possibility.

Trailing cold front anchored along the elongating height fall/shear 
axis lifts through early tonight /02z-06z/.  Period of stronger dcva 
within the emerging cold air advection will sustain the potential 
for shower production through this time, particularly with westward 
extent. Advent of isentropic descent coincident with a tightening 
gradient and corresponding uptick in wind magnitude through the 850-
925 mb layer will allow for gusty conditions immediately behind the 
cold frontal passage.  Brief duration of gusts into the 35 to 40 mph 
range not out of the question, limited mainly by modest mixed layer 
depth.  

Deep longwave troughing will then govern conditions through the 
Tuesday-Wednesday periods, anchored by the consolidation of the two 
separate negative height anomalies over the next 36 hours.  A 
secondary period of cold air advection on Tuesday as upper heights 
continue to fall. This will lower 850 mb temperatures to near zero 
by afternoon. This process into a background of sustained cold/moist 
cyclonic flow will maintain the chance of rain. A few instances of 
graupel a possibility given the steep low level lapse rates and 
suppressed freezing levels. Brisk conditions expected with southwest 
winds turning quite gusty as late day diurnal processes augment a 
tight pressure gradient. High confidence exists for gusts to 30 mph, 
with moderate potential for a period of gusts over 35 mph from mid 
afternoon into the evening.  Highs low-mid 50s. 

Growing overlake convective depth as instability increases within 
this colder air mass will initiate a period of greater downstream 
moisture flux off Lake Michigan under northwest to westerly flow 
Tuesday night and Wednesday.  This will sustain a substantial low 
stratus canopy, while maintaining the chance of rain.  Greatest shot 
for rain may center on Tuesday night, with general model consensus 
depicting a deeper plume of moisture tied to a weak trough or 
secondary frontal passage.  Coldest day of the fall season thus far 
expected on Wednesday, with limited diurnal recovery holding highs 
largely in the 40s.

Transient upper level ridging will build into the region on Thursday 
bringing dry conditions as highs increase to the upper 50s. 
Southwesterly flow will then increase late Thursday into Friday 
ahead of the next low pressure system organizing over the Northern 
Plains. As the low tracks through the Northern Great Lakes, a cold 
front will push through the region with a period of showers 
accompanying the frontal passage. The 12Z GFS and Canadian solutions 
continue to advertise an earlier frontal passage late Friday while 
the 12Z Euro favors a later frontal passage on Saturday. The main 
difference between these solutions is the location of an amplifying 
upper level trough and associated jet streak with the Euro keeping 
the trough axis just west of the region on Saturday. Regardless, 
upper level troughing looks to build into the region for the 
remainder of the weekend as highs fall into the upper 40s with the 
first shot at widespread low temperatures below freezing on Sunday 
morning.

&&

.MARINE...

Gales expected across Lake Huron tonight through Tuesday night, and 
Lake St. Clair and the western shores of Lake Erie late Tuesday 
through Tuesday night...

A strong low pressure system will continue to move north into lower 
Michigan through this evening, before rapidly deepening tonight as 
it lifts further northward and stalls near the Straits by early 
Tuesday morning. Model guidance is in fairly good agreement that 
this area of low pressure will deepen further to around 980 hPa as 
it lingers near the Straits through Tuesday night, before exiting to 
the northeast towards Hudson Bay by Wednesday. The strong low 
pressure system will bring gale-force winds to the local waters, 
especially Tuesday into Tuesday night as cold air wrapping around 
the system enhances instability over the lakes, when probabilistic 
guidance continues showing increasing confidence in 40-45 knot 
gusts. Wind direction will initially be southeast through this 
evening, before becoming south/southwest Tuesday into Wednesday. 

Wave heights will peak around 10-15 feet in the open Lake Huron 
waters, with waves of 3-8 feet in Saginaw Bay and the nearshore 
waters of the Thumb, and 2-4 feet across Lake St. Clair and western 
Lake Erie. Winds and waves will rapidly begin to diminish starting 
Wednesday as the low pressure system exits/weakens and brief high 
pressure builds into the region.

&&

.HYDROLOGY...

Strong low pressure system will move just to the north and west of 
southeast Michigan through tonight, and stall near the Straits 
through Tuesday before exiting to the northeast Wednesday. Model 
guidance continues to show the heaviest rainfall axis being located 
primarily west of the US 23/I-75 corridor. The Tri-Cities area still 
looks to see the highest rainfall totals, generally 1-2 inches 
through Tuesday morning, with an inch or less across the rest of 
southeast Michigan. This rainfall will fall over a prolonged period 
of time, and combined with the progressive nature of the low 
pressure system, no major areal flooding concerns are anticipated. 
Regardless, a Flood Watch remains in effect for Midland and Bay 
counties through tonight, for localized higher potential rainfall 
totals.

Behind the departing low pressure system, rising river levels may 
result in minor river flooding in the Tri-Cities area (particularly 
the Saginaw River at Saginaw).

&&

.PREV DISCUSSION...
Issued at 136 PM EDT Mon Oct 23 2017 

AVIATION...

Flying conditions will continue to tutored through the evening 
hours as deepening low pressure system approaches from the lower 
Ohio River Valley and interacts with a lingering frontal boundary 
draped across north-central Michigan. Continued surge of low-level 
moisture will accompany the approaching low, with occasional 
showers continuing through the evening hours, most persistent at 
KMBS due to proximity to lingering frontal boundary. Best chance for 
heavier rain at the remaining TAF sites looks to be between 18z-02z, 
before activity gradually becomes more scattered in nature overnight 
as low pressure lifts just to the north and west through central 
Michigan. MVFR/IFR ceilings and occasional visibilities will 
accompany the rain through this evening, with probabilistic guidance 
showing best chance for IFR ceilings late this afternoon and into 
this evening, before slowly improving to low-end MVFR as rain tapers 
off after midnight. Low pressure will rapidly deepen into tonight, 
resulting in increasing winds. With track just to the west of the 
terminals, enough stability in the low-levels should keep gusts 
limited to around 25 knots, initially out of the southeast before 
becoming south/southwest around midnight. Gusts of 20-25 knots will 
continue through the remainder of the period with scattered showers 
again possible after 12z.

For DTW...Moisture surge will result in best period of rainfall, 
some possibly heavy at times, between 18z-02z, before rain slowly 
tapers off through the night as deepening low pressure pulls away to 
the north and west. MVFR/IFR ceilings will be possible in rain 
through much of the night, with occasional reductions in visibility 
as well. Ceilings remain at or below 5 kft through the period. Low 
confidence that developing southeast gusts to 25 knots this evening 
pose runway configuration issues, before more favorable conditions 
by midnight as winds veer to the southwest, but remain gusting to 25 
knots.

//DTW THRESHOLD PROBABILITIES...  

* High in ceilings at or below 5000 ft through the period. 

* Low in southeast winds exceeding crosswind thresholds this 
  evening.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch through Tuesday morning for MIZ047-048.

Lake Huron...Gale Warning from 8 PM this evening to 8 AM EDT Wednesday for 
     LHZ362-363-462>464.

     Gale Warning from 8 PM this evening to 10 PM EDT Tuesday for 
     LHZ441>443.

     Gale Warning from 1 AM to 8 PM EDT Tuesday for LHZ361.

     Gale Warning from 2 AM to 10 PM EDT Tuesday for LHZ421-422.

Lake St Clair...Small Craft Advisory until 2 PM EDT Tuesday for LCZ460.

     Gale Warning from 2 PM Tuesday to 4 AM EDT Wednesday for LCZ460.

Michigan waters of Lake Erie...Gale Warning from 2 PM Tuesday to 4 AM EDT Wednesday for LEZ444.

     Small Craft Advisory until 2 PM EDT Tuesday for LEZ444.

&&

$$

DISCUSSION...MR/JD
MARINE.......IRL
HYDROLOGY....IRL

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