NWS Forecast Discussion

FXUS63 KDTX 201134

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
734 AM EDT Fri Jul 20 2018


Broad and slow-moving low pressure system will meander near Lake
Michigan today, with increasingly moist and unstable airmass
overspreading the airspace as the day progresses. Early this
morning, high debris clouds will continue to fill in across the
region, with diurnal heating supporting likely robust cu field in
the 5-7 kft range heading into the afternoon. Big concern this TAF
cycle is if/when on the occurrence of thunderstorms mid afternoon
through the evening hours. Hi-res guidance has started to come in
slightly better focus on highest confidence timing window (22z-02z),
but signal still remains noisy with the best chance of thunderstorms
occurring at KPTK, KDET, KDTW, and KYIP. Further complicating things
will be the potential for severe storms, further enhancing erratic
wind gusts associated with stronger thunderstorms. Convection
continues into the overnight but as primarily periodic MVFR showers
with embedded thunder. Southeast winds will become gusty at times
today peaking at 20-22 knots.

For DTW...Highest confidence for TEMPO thunderstorm group 22z-02z.
Potential exists during this period for rather robust cluster/line
of storms to move across or near the terminal with brief IFR
conditions. Southeast flow will favor southwest runway operations,
with a low potential for east runway operations late this evening.


* Moderate in cigs aob 5000 feet 16z-00z. High after 00z.

* Moderate in thunderstorm occurrence and timing late Friday
  afternoon and evening. Low overnight.


Issued at 407 AM EDT Fri Jul 20 2018


Main item to contend with the next few days is the closed upper
level low pressure system that is centered over the Upper
Mississippi River Valley. Atypical to see this configuration of
upper level jet streak in mid summer, piercing the upper Midwest and
accompanied by strength/integrity of potential vorticity anomaly
attendant to strong left exit region. NWP consensus shows low
pressure peaking 12Z this morning over western Wisconsin (H5 geo
heights of 571dam), holding largely steady today, beginning to fill
tonight/Saturday. Relative strength of dynamical support for July,
gives plenty of discuss.

Today...Strengthening south southeasterly flow has allowed surface
moisture to return the past couple of hours with sfc dewpoints now
reaching 60 degrees. Midlevel moisture is still lacking at this
early hour, but plan view perspective shows rapid moisture return in
the 850-600mb layer by 15Z. Initial arc of accas and elevated
showers over far western Lower Michigan appears tied to 875mb
moisture. Not much else going in the near term and will keep the
forecast dry ahead of 12Z. Aforementioned deep midlevel theta e axis
arrives late morning and afternoon bringing increasing chances for
showers and thunderstorm development. Saturation of the 2.0 to 6.5
kft agl layer will allow SBCAPE/MLCAPE to increase particularity
during the late afternoon hours. Latest projections suggest that as
much as 1000 J/kg to potentially 1500 J/kg will be feasible. Cloud
cover may become a concern, however, quality of moisture lends some
confidence in the instability. What is really a question is amount
of shear that will exist this afternoon over Southeast Michigan. At
first glance, one notices integrity of backed southeasterly surface
flow, but little directional shear will exist in the midlevels which
significantly compromises hodograph length. 0-3km SRH is expected to
remain less than 100 m2/s2 for the majority of Southeast
Michigan with 0-6km bulk shear generally between 15-20 knots.

Attention will be on far Southwest Michigan 17-19Z timeframe as an
arc of thunderstorms are expected to initiate. Consensus of CAMS
show this thunderstorm activity struggling to lift northeastward
given mean flow, losing intensity west of the cwa between 22-01Z.
The severe thunderstorm threat for today largely appears tied to
this thunderstorm activity that will originate over SW Michigan.
Really a conundrum with how the dynamics are set up. Models show a
period of outstanding direct PV advection, however, system relative
isentropic ascent will be poor given unidirectional flow in the
lowest 10 kft agl. It remains possible that a favorable
shear/instability balance will be struck with some thunderstorm
activity bringing an isolated severe threat. Latest Swody1 has much
of Southeast Michigan in a Slight Risk designation. The most viable
threats being damaging wind gusts to 60 mph and large hail to 1 inch.

Given latest MOS, increased high temperatures today a couple of
degrees across the board. Additionally, with precise wind direction
today out of the south southeast, decided on issuing a Lakeshore
Flood Advisory for Wayne and Monroe Counties. Not entirely sure how
it will unfold, but persistence of wind direction and sustained wind
speeds feeling is that water levels will become impactful.

This evening...Really an interesting little setup for the eastern
Thumb this evening as combination of lead edge to rich low level
theta e axis and lowering mixing heights due to evening transition
may allow convective instability to increase somewhat rapidly. A
subset of CAMS are showing some convective initiation near the far
southern Lake Huron basin before lifting northwestward into the
eastern Thumb 2-6Z timeframe. Forecasts show a localized region with
cyclonically curved hodographs and SRH values exceeding 100 m2/s2.
There is a large variance in vigor of convection amongst the CAMS,
so confidence is low.

Saturday...Elongated upper level low pressure is forecasted to
center over portions of southern Lower Michigan and northern IN/OH.
Proximity of the upper level low will keep the forecast under deep
column theta e. Forecast soundings show a moist sounding with weak
unstable profile and moist adiabatic lapse rates. Pwats are
forecasted to range around 1.75 inches. Given the combination of
daytime heating with various shreds of embedded shortwave energy,
looking at a numerous coverage of showers and thunderstorms
throughout the daytime Saturday. Not looking at an all day washout,
but the narrative is one of intermittent, nuisance shower/tstorms.
No severe weather threat anticipated Saturday.

Sunday...A significant amount of 6-15 kft agl layer drying is
forecasted Sunday with the dry air actually arriving from the east.
In addition to this, there is some signal for active subsidence
taking root between 3.0 and 6.0 kft agl which will yield fairly high
stability. For this reason, Sunday is considered the much better day
for this weekend with highs rising to the upper 70s to around 80

Early next week...A very complex upper level flow pattern is
expected to develop with a numerous areas of strong blocking. Latest
indications suggest that Southeast Michigan will remain within a
weak surface ridge. Relatively quiet weather.


A broad area of low pressure will slowly meander around the Great
Lakes today and continuing through much of the weekend. Periodic
showers and thunderstorms will accompany the low. The low will
slowly drift southeastward with time into the northern Ohio River
Valley by Monday. Southeast winds will prevail across the local
waters through most of Saturday, with gusts peaking at 20-25 knots
across much of the local waters. The exception will be across
western Lake Erie, where gusts of 25-30 knots will be possible and
will pose hazardous conditions for small craft. The increased winds
will also lead to hazardous small craft conditions from Port Austin
to Port Huron due to elevated waves along the Lake Huron nearshore
waters. Winds will shift to the northeast and slowly decrease Sunday
and continuing into Monday as the low pressure drifts further south.
Drier conditions and improved marine conditions will then prevail
early next week.


A slow-moving and broad low pressure system meandering around the
Great Lakes will bring periodic showers and thunderstorms across the
region today and through most of the weekend. Rainfall amounts will
likely be highly variable with coverage more scattered than
widespread. Amounts will range from around a quarter of an inch to
around 1 inch, with locally higher amounts possible associated with
stronger thunderstorms. Recent dry conditions will preclude any
flooding threat for most, with the exception being minor poor
drainage flooding associated with stronger thunderstorm activity.
Conditions will begin to dry out late this weekend as the low
pressure system drifts southeast into the northern Ohio River


MI...Beach Hazards Statement through this evening for MIZ049-055-083.

     Lakeshore Flood Advisory from 2 PM this afternoon to 4 AM EDT
     Saturday for MIZ076-083.

Lake Huron...Small Craft Advisory until 4 AM EDT Saturday for LHZ441>443.

Lake St Clair...NONE.
Michigan waters of Lake Erie...Small Craft Advisory until 4 AM EDT Saturday for LEZ444.




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NWS DTX Office Area Forecast Discussion