Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 09/23/21
Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1203 AM EDT Thu Sep 23 2021
No big changes to expectations on flight categories late tonight and
through the day. The surface low continues backing west from central
Lake Erie into SE MI but on a slightly farther north track. This
leads to some minor adjustments on wind direction forecasts across
the terminal corridor, mainly south of FNT. Otherwise, the last
surge of heavy rainfall unfolds for a few hours after midnight and
then weakens and shifts into western Lower MI by sunrise. Showers
are then replaced by drizzle and fog in the weaker pressure gradient
around the surface low center. IFR/LIFR ceiling and mostly IFR
visibility becomes widespread across the terminal corridor with just
some minor improvement to high end IFR by Thursday evening.
For DTW... Northerly wind gusts ease as the surface low moves nearly
overhead by early morning. As the low approaches, primarily MVFR
conditions drop to IFR with LIFR likely in drizzle, fog, and low
clouds. Some improvement occurs by early afternoon as the low drifts
north of the terminal area into Thursday evening.
.DTW THRESHOLD PROBABILITIES...
* High for ceiling 5000 feet or less through the period.
* Moderate for ceiling 200 feet and/or visibility 1/2 SM late
tonight until early afternoon.
Issued at 1023 PM EDT Wed Sep 22 2021
One final phase of this rain event keeps the Flood Watch going
tonight while the strong surface low in central Lake Erie has the
peak of the wind event in progress. The Flood Watch and Wind
Advisory/Warning continue through tonight with expiration times
remaining on schedule as conditions improve early in the morning.
A relative break in rain activity occurred during the evening
between the primary moisture axis and the deformation/trowal within
the upper low. This provided a welcome breather for run-off to occur
before the next batch of showers moving in from IN/OH at press time.
Observations support model guidance that indicates the upper low has
plenty of moisture and dynamics to wring out another 1 to 2 inches of
rain during the night before closing and pinching off the moisture
axis mid to late morning.
The wind was the other high impact aspect of this event which really
got going along the Lake Huron shoreline early to mid evening. The
latest runs of RAP model soundings indicate a well mixed boundary
layer up to 850 mb at points along the shoreline as of 00Z which
continue 50 knot potential through nearly 06Z before diminishing as
the low backs into SE Michigan during early morning.
Issued at 339 PM EDT Wed Sep 22 2021
A Flood Watch remains in effect for most of Southeastern Michigan
through tonight. A Wind Advisory and Lakeshore Flood Advisory also
remain in place from parts of the Saginaw Valley into the Thumb
where strong onshore flow will persist (and further strengthen) into
Rain continues to stream northward through the area this afternoon
within an area of very strong upper level divergence as upper vort
lobe lifts into the area in advance of deepening upper low pressure
over Indiana. Expect periods of light to moderate rain to prevail
into the early evening hours with some pockets of heavy rain likely
as well as moisture plume feeding around this developing system
Meanwhile, surface cyclo-genesis is ongoing over central/eastern
Ohio in advance of main upper system. This low pressure will
strengthen into tonight and pull back northwest into far western
Lake Erie/far SE Lower Michigan as upper low deepens considerably as
strong jet (80 kts at H5/100+ at H3) pivots northeast into the
region around its base. This process will lead to a strong increase
in FGEN forcing which will pivot north/northwest into the region
later this evening into the overnight. Upper level divergence will
remain in place to further enhance lift and pockets of elevated
instability will also ride back NNW up the frontal slope over the
region as this system quickly matures.
All of these factors favor a continuation of rain, heavy at
times, after perhaps just a brief lull early this evening between
this afternoon`s vorticity lobe and the main system tonight. In
fact, would expect rainfall rates to increase as the strongest
forcing/lift develop over the region late this evening into parts of
the overnight. Convective elements within the rain shield will also
aid in higher rates during this period of time. Additional rainfall
of 1 to locally 2 inches will be likely from late today into tonight.
As the deepening surface cyclone pivots back towards the area, expect
a tightening pressure gradient as well with very gusty north to
northeast winds. This will be especially true downwind of Lake Huron
where a Wind Advisory will remain in effect through tonight as wind
gusts to 45 to 50 mph are expected along/near the shoreline. These
strong onshore winds will also lead to some lakeshore flooding and
erosion concerns with a Lakeshore Flood Advisory also remaining in
place through tonight. Further inland, wind gusts of 30 to 40 mph
will be common over much of the rest of the forecast area.
As the upper system matures/occludes tonight as it lifts north over
the region, mid level dry air will work into the system and lead to
a steady downtrend in rainfall rates/coverage later tonight into
early Thursday morning. That said, periods of rain will persist
into/through Thursday as the low pressure, basically vertically
stacked to 150 mb, lifts gradually north/northeast across the region.
After a cool day in the 50s again Thursday, transient shortwave
ridging will work into the area on Friday and bring dry conditions
will high temperatures back to near 70. A vigorous shortwave and
attending cold front will then surge through the area late Friday
night into Saturday. This will bring the chance of scattered showers
and cooler conditions back into the 60s Saturday with cooler
conditions then persisting into Sunday before heights gradually
build early next week on the southwest periphery of upper troughing
over eastern Canada. This should allow 70s to return to the forecast
for the Monday/Tuesday time frame for at least portions of the area.
Low pressure continues north this evening preserving a constricted
pressure gradient. Numerous surface obs are currently gusting to
gales, and some sustained gales are expected upon arrival of the
strongest gradient winds late this evening. Upped peak gusts by a
couple knots based on latest guidance, but still comfortably below
storms (50 knots) across the central Great Lakes. Also made minor
timing adjustments to the Gale Warning, ending earlier (2 AM EDT)
for western Lake Erie and Lake St. Clair since the strongest winds
should clear north shortly after midnight. Gales are still marginal
for northern Huron, but the southern fringe of the zone should see a
period of even gales, thus kept the northern most marine zone in
with this update. Otherwise, the current headline remains on-track.
Occasional waves peak near 20 feet late tonight, mainly for the
south-central Huron basin. Winds drop-off quickly mid-morning
Thursday, but residual wave energy will likely warrant a back-end
Small Craft Advisory for the southern Huron nearshores until the
afternoon. Weaker yet modest W/SW winds expected behind the low
through the weekend.
Periods of rain will continue as an upper low pressure deepens to
the southwest. As this storm system lifts into the region tonight,
expect areas of heavy rain to become more common this evening with
this activity persisting into part of the overnight. An embedded
rumble of thunder or two can also be expected. Additional rainfall
by early Thursday morning should average generally around one inch
with local 2 inch amounts possible. This rainfall will compound
flooding on area rivers that is already underway due to the heavy
rainfall that has fallen to this point. Some areas of urban and
poor drainage flooding can also be expected with the most notable
flooding within the drainage basins of the various branches of the
Clinton and Rouge Rivers
MI...Flood Watch until 8 AM EDT Thursday for MIZ049-053>055-060>063-
High Wind Warning until 8 AM EDT Thursday for MIZ049-055-063.
Lakeshore Flood Advisory until 8 AM EDT Thursday for MIZ048-049-054-
Wind Advisory until 8 AM EDT Thursday for MIZ048-054.
Lake Huron...Gale Warning until 8 AM EDT Thursday for LHZ361>363-421-422-462>464.
Storm Warning until 8 AM EDT Thursday for LHZ441>443.
Lake St Clair...Gale Warning until 2 AM EDT Thursday for LCZ460.
Michigan waters of Lake Erie...Gale Warning until 2 AM EDT Thursday for LEZ444.
You can obtain your latest National Weather Service forecasts online
Area Forecast Discussion
National Weather Service Gray ME
1133 PM EDT Wed Sep 22 2021
Warm and humid conditions will persist across the region through the
end of the week as southerly winds continue ahead of a slow moving
cold front. This front will cross the region late Friday and into
Saturday with a period of widespread rainfall likely along with a
few thunderstorms. Improving conditions are expected by the second
half of the weekend as drier air arrives from the west. A cold
front will then approach the region early next week with another
threat for showers.
.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
Will continue with the chance of showers or patchy light rain across
the region late this evening, mainly over New Hampshire where
persistent echoes continue to enter the state per latest radar
imagery. Some light precipitation will continue in the mountains
and foothills of Maine as well. Made minor adjustments to
current temperatures, dew points and winds. Some patchy drizzle
and fog will continue to form after midnight per latest HREF and
HRRR solutions and observational trends.
High Impact Weather Potential: Patchy dense fog is possible
Pattern: The trough/ridge pattern across central/eastern North
America has slowly shifted east over the past few days with the
longwave trough axis now moving into the Great Lakes region with the
downstream ridge now just east of the US East Coast. Between these
two features is a deep plume of moisture...evident on water vapor
imagery...streaming north from the Gulf of Mexico up along the
Appalachians west of New England. In our area...a prolonged period
of southerly flow has yielded a large amount of stratus...along with
some drizzle and fog along with a few rain showers. More of the
same is expected through tonight with little change to the overall
flow configuration. Thus...forecast challenges center on the
evolution of fog/stratus and whether we`ll see any dense fog
across the forecast area.
Through this evening: Weak vort lobe in the southerly flow aloft has
brought a modest increase in shower coverage over New Hampshire this
afternoon. These will continue to lift north and weaken through the
evening leaving us with ample low cloudiness that will thicken for
all but far northern areas. It will remain mild...with 8pm
temperatures still in the 60s throughout the CWA.
Tonight: No real change to the pattern through tonight with a repeat
of low clouds...drizzle...and some fog expected from the mountains
south. Short range guidance paints another weak band of enhanced
vorticity aloft pushing north and west through the forecast area
overnight. This may help to bring a few more showers...otherwise the
best chance for shower activity will remain along the southern
slopes of the mountains. Higher dewpoints than 24 hours ago
suggests that we`ll see more in the way of fog...with locally
dense fog possible with the strongest signal for this being over
the foothills. Given the continued moisture advection...expect
lows to take another step upward with upper 50s in the mountains
and lower 60s to the south.
.SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
High Impact Weather Potential: Additional patchy dense fog possible
Pattern: A potent shortwave dropping into the Northern Plains will
act to take the trough/cutoff low to our west and kick it
north...with little room to move downstream given a high over low
/Rex block/ setup just off the east coast. Through the short term
forecast period the trough...as well as the collocated occluded
surface low will move north from a position near Detroit Thursday
morning to a position over Georgian Bay by Friday morning. East of
this...a slowly advancing cold front associated with an atmospheric
river will slowly march east...nearing our forecast area by the very
end of the short term period. With moist...southerly flow
continuing ahead of this advancing cold front forecast concerns will
continue to be centered on fog...stratus...drizzle...and the
potential for showers.
Thursday: H5 555DM low will slowly move north through Michigan
during the day out with a 590DM high centered just south of Nova
Scotia. Warm/moist southerly flow will continue with T9s sitting
around the +17C mark with PWATs still not all that impressive
/nearing 1"/ given deep dry wedge overtopping the surface-based
moist layer. Like today...expect we/ll see improving visibilities
and ceilings during the day...but another mostly cloudy day is
expected with some showers in the mountains given upslope flow.
Temperatures aloft really won/t be that different than 24 hours
previous...so expect 70s to dominate again.
Thursday Night: There is rather strong ensemble agreement that the
frontal boundary will remain west of the CT Valley through daybreak
Friday which means the potential for any heavier rainfall will hold
off until the beginning of the long term forecast. The southerly/
southeasterly low level jet will gradually strengthen overnight
which...given continued moist upslope should allow for the upslope
showers in the mountains. Still...the moisture looks to be
relatively shallow...perhaps 5kft...with a robust dry wedge above
this with impressively high heights at H5...nearing the 99th
percentile for this time of year. Statistical guidance is a bit
more robust with the fog during this period than the previous
night...but there is little support for widespread dense fog and
will therefore just continue patchy fog mention...particularly over
coastal ME into the foothills given the best marine influence.
.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Starting with a global view we`ll have a higher amplitude and
blockier pattern developing over North America as the colder polar
air starts its seasonal slip southward. By the end of the week an
upper level low over the Great Lakes will begin to cut off while
high pressure noses east of us into the Gulf of St. Lawrence. These
two features will provide the alignment for a feed of southerly
moisture to start the weekend. By the end of the weekend, expect
there to be some progression with the low pulling north and a more
open wave developing. This should allow systems to continue to remain
progressive through the start of next week.
Focus of this forecast will be the Friday-Saturday time frame. Here
a surface low will occlude over the Great Lakes with an extended
warm sector out ahead of it. The cold front will serve to focus the
southerly flow of moisture into the region, setting us up for at
Atmospheric river with heavy rainfall possible. At this point the
timing of this system is relatively secure, with all the major
deterministic and ensembles showing a period of rain from Friday
morning through to Saturday night. This agreement is reflected in
the high PoPs issued with this package.
Rainfall amounts are a bit sticker. Southerly flow and PWAT values
of over 1.6" result in net moisture transport of > 750 which is
enough to be considered an `atmospheric river`. With the system
wrapping up tend to favor a slightly more conservative take on
the total QPF, so while there are a few extreme options out
there feel sticking closer to the mean is more reasonable.
Thunderstorms will also be a threat Friday afternoon. Forecast
soundings show significant low level instability with CAPE upwards
of 1000 J/kg. Additionally there is a 40kts low level jet which will
provide for plenty of shear and turning hodographs. Along the ME
coast, saturated low levels will keep things in the fog with only
elevated convection, however across New Hampshire thunder is
likely as the front moves through and have included this in my
grids. The same low level jet which will increase the shear will
also allow for training of thunderstorms along the front which
could make for locally higher rainfall.
By Saturday the consensus is leaning towards precipitation
departing the region and have kept PoPs a bit lower through evening,
although some showers are likely to linger. The low pressure aloft
will linger to our west with another system moving through late in
the weekend. Here timing differences start to appear amongst the
guidance with the GFS favoring a more progressive solution and the
Euro and CMC lagging behind. Have slightly favored the slower
solution here which keeps the upper low overhead and showers in the
area, especially the mountains through the start of next week.
.AVIATION /04Z THURSDAY THROUGH MONDAY/...
Summary: Moist southerly flow will cause an extended period of low
clouds...fog...and some drizzle that will continue through Thursday
night with some improvement each afternoon.
Restrictions: Conditions improving to VFR/MVFR attm but showers are
moving north toward southern New Hampshire which...will begin to
bring deteriorating conditions here...with further deterioration to
LIFR/IFR tonight /outside of HIE/ as stratus re-solidifies and
fog/drizzle develop. Expect this cycle to occur again on Thursday
with improving conditions during the day before another round of
LIFR/IFR Thursday night with conditions possibly even a bit worse
Thursday night than tonight.
Winds: Southerly winds are strengthening to around 10kts inland and
10g18kts along the coast. These will diminish to less than 5kts
overnight before increasing to around 10kts with some higher gusts
from the southeast for the day on Thursday. 5-10kt southeasterly
winds are expected to continue Thursday night.
Lightning: Lightning is not expected through Thursday night.
Low Level Wind Shear / LLWS: No LLWS is expected through Thursday.
Thursday night...low level southeasterly winds will increase with a
period of LLWS likely.
Long Term... High impact weather potential for Friday. Along the
Maine coast IFR with fog and low stratus will likely linger
through most of the day. Further east across New Hampshire
clearing will allow for instability to develop and
thunderstorms with heavy rain will cross the area from west to
east impacting all portions of the CWA. Additionally a low level
jet may result in a brief period of low level wind shear as the
front moves through. By Saturday conditions will improve with
just lingering MVFR showers through the weekend.
Short Term...Wind gusts have pushed towards 20kts this
afternoon from the southeast...but are expected to diminish
some this evening. Expect marine fog to be on the increase
tonight through Thursday as surface dewpoints rise...with
continued southerly winds bringing gradually increasing wave
heights. Winds and waves begin to approach SCA levels as this
forecast period ends Thursday night.
Southerly flow will increase on Wednesday ahead of a cold front
crossing the Gulf of Maine and a few gusts to SCA levels are
possible. The front will clear through on Saturday with winds
and seas remaining calm through the start of next week.
Area Forecast Discussion
National Weather Service Reno NV
146 PM PDT Wed Sep 22 2021
A slight cooling trend starts overnight into tomorrow with daytime
highs hovering just below seasonal averages. Temperatures rebound
slightly by Friday and continue with slightly above average values
through the weekend. Areas of haze and smoke should be fairly
transient as winds diminish this evening before turning mostly to
the north and east Thursday morning through Friday.
The main weather highlights this first day of Fall through this
weekend will be an increasing easterly low and mid-level flow and
reduction in smoke and haze across most areas of the eastern Sierra
and far western NV. Slightly cooler temperatures tomorrow but
rebounding going into the weekend.
High pressure will nose east into the Sierra and western NV this
evening into tomorrow as a weak cold frontal boundary exits east
into the Rockies. The resulting west-east orientated upper ridge
axis will push over across central OR into southern ID. This will
result in a mostly northeasterly flow sitting up across western NV
and the Sierra that continues into northern and central CA. As
this flow pattern continues into Friday, smoke and haze from
regional fires will decrease sharply and be directed mostly east
of the Sierra Crest. Hi-Res HRRR simulations complement this
forecast outcome for the near-term forecast into Friday.
By this weekend, the ridge axis will gradually weaken and sag south
thus allowing a more westerly mid and upper flow to increase across
the region. The resulting upper height falls will steepen thermal
gradients across western NV Basins that translate to a return of the
typical afternoon zephyr breezes with gusts to 25 mph over the
eastern Sierra and far western NV. Unfortunately periods of haze and
smoke will once again begin to filter east of the Sierra crest into
western NV. But this is all contingent upon regional fire activity.
As the upper ridge recedes westward, warm and dry conditions will
continue influence the region through this forecast period.
Highs tomorrow will be cooler and only rise into the 70s to lower
80s for valleys as cooler air spreads south and southeast over the
region in an increasing easterly wind regime. Highs on Friday will
rebound several degrees into mostly 80s across western NV valleys
with higher Sierra valleys edging up into the upper 70s.
EXTENDED OUTLOOK through the middle of next week...
* CHANGES: Another cooling trend for next Tuesday-Wednesday with
possible precipitation over northern portions of CA/NV.
Ensemble cluster and extended model guidance are beginning to show a
pattern change by mid-week next week. Any precipitation that
results from this anticipated pattern change will be mostly light
and confined to the northern Sierra. But it is worth noting that
the second week of the autumn season has a long-wave trough
pattern digging south across the West Coast before surging bursts
of short wave energy into the region. The only nexus will be a
split trough pattern separating upper dynamics and moisture into a
cooler more dynamic northern branch and a much drier southern
extension into the Sierra and Northern NV. Therefore, it is safe
to say that forecast confidence is split between a higher much
cooler outcome that is accompanied by a drier, more uncertain
rainfall prediction over the forecast region. The best bet lies in
the increasing probability of gusty conditions Monday-Tuesday as
both European and GEFS ensemble guidance show tightening gradients
as the upper trough deepens over the western US. One thing is for
sure, the upcoming pattern change could reflect in today`s season
For today, gusty southwest winds to 20-25 kts, locally to 30 kts,
are likely in the afternoon and evening mainly north of US-50. Light
to moderate turbulence is expected to increase along and east of the
northern Sierra crest due to the increased airflow. Increasing winds
this afternoon should mix out most smoke affecting area terminals.
Winds turn to easterly Thursday for a shift of any turbulence to
over and west of the Sierra Crest. Lighter winds and dry conditions
expected through the weekend, until the next low pressure system
moves into the region Monday with increasing winds and chances for
* Breezy winds today may lead to localized areas of critical fire
Breezy winds remain on track as a system passes to our north today.
Expect gusts in the 25-35 mph range during the afternoon and evening
hours. In the higher elevations of the Sierra and northeast
California, peak ridge level winds may touch 45 mph. The combination
of the wind with dry conditions may produce localized areas of
critical fire weather conditions especially the Sierra Front for a
After today, winds switch out of the north and east. Winds will be
lighter for most areas, but some areas over the Sierra Crest along
the western slopes will become breezy early Thursday morning and
again Thursday night into Friday morning. -Hoon
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