Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/29/19
Area Forecast Discussion
National Weather Service Key West FL
947 PM EDT Wed Aug 28 2019
DISCUSSION...In the middle and upper levels(700-200 MB), latest
available satellite imagery overlaid with model streamline
analysis as of 900 pm, like 24 hours prior, depicts a complex
middle and upper level flow pattern over the Southeastern United
States including Florida and the Florida Keys, with the axis of a
east to west oriented ridge above 500 mb extending in the Central
Gulf from along 90W to across to across Central Florida thence
northeast to near 30N 75W. East of that feature, a (TUTT)
tropical upper tropospheric trough cell is migrating slowly
westward and is now located just north of the Dominican Republic
near 22N 70W. Of course, southeast of that feature is
strengthening Hurricane Dorian, which was located at 8 pm to be
at 19.2N 65.7W or about 60 miles NNE of San Juan, Puerto Rico.
At the surface and in the middle levels(Surface to 700 mb), latest
available marine and land surface observations and analysis as of
900 pm, details decoupled Tropical Depression #6 which at 5 pm was
located about 165 miles ESE of Cape Hatteras, NC. This system
trails a trough axis south southwestward into the Central Bahamas.
As was the case 24 hours ago, there is a near 1015 mb meso- hi
pressure center located over the SE Gulf of Mexico. Latest CIRA
data suggests deep moisture, typical at this time of year,
surrounds the Florida Keys with PWAT at 1.75 to 2.00 inches, but
rises to between 2.00 and 2.25 inches upstream of the Keys across
all of the Northern and Central Bahamas in association with
aforementioned approaching TUTT cell and the low level trough. The
evening sounding illustrated PWAT at about 1.80 inches with still
gentle to moderate northwest to north flow from off of the surface
up to 500 mb, then continuing to be light to gentle all the way to
.CURRENTLY...As of 900 pm, skies are partly cloudy across all of
the Florida Keys and surrounding waters. Radar still detects
isolated to widely scattered showers and isolated storms still
exist along boundaries, mainly concentrated attm across the
Eastern Florida Straits, but like last night, this was the
remnants of clusters of showers and storms that moved off the
Mainland from the seabreeze in the moist gentle to moderate
northwesterly flow. Temperatures across the islands remain in the
upper 80s with sticky dewpoints in the upper 70s. C-man stations
along the Florida Reef are still recording NW winds between 10 and
15 knots, but with gusts between 15 and 20 knots. Island sensors
are recording NW winds between 10 and 15 mph.
.SHORT TERM...Overnight and Thursday, as the trough over the
Bahamas continues to slide westward overnight towards the Keys, increasing
moisture upstream of the Keys and surrounding waters will work
its way westward, with PWAT rising to between 2.00 and 2.25
inches by 12Z. As such, HRRR and the Caribbean HRRR both agree
additional showers and storms will redevelop later tonight again
along boundaries upstream of both the Upper Florida Keys, as well
as over the Extreme Southeastern Gulf of Mexico upstream of both
the Middle and Lower Keys. The configuration of the ridge to our
southwest and the approaching surface/low level trough will
continue to allow for gentle to moderate winds this evening and
overnight and early Thursday morning. The confluent flow will
drive boundaries, showers and storms across the Keys and all
surrounding waters. As such, we are carrying 50% chances for 12
hour pops for scattered showers and storms, above the 35% climo
pops for the date.
.MARINE...Gentle to moderate breezes will continue overnight and
Thursday morning. Winds and seas will be higher in and near some
of these showers and storms. Hurricane Dorian is expected to
track northwest overnight through Friday morning.
.AVIATION...Mainly VFR conditions will prevail at the island
terminals for the next 24 hours, but there will be periods of
shower and thunderstorm activity near each terminal. For example,
convection will blossom over the Southeast Gulf after midnight and
then move to the southeast toward EYW and MTH during the late
night and morning hours. These showers should dissipate by mid-day
Thursday, and the overall trend is reflected with VCSH in the
TAFs. Then afternoon thunderstorms over the Everglades on Thu
afternoon will slide to the southwest toward the Middle and
possibly the Lower Keys during the late afternoon and early
.TROPICAL...Now is the time to make sure you have a hurricane
plan. Now is the time to have your supplies ready. Do not wait
to make a plan and gather supplies when under increased stress.
Portions of the Florida Keys are presently inside the forecast
cone as of 900 pm. Remain up-to-date with the latest forecast
either through hurricanes.gov or weather.gov/key.
At 800 PM AST (0000 UTC), the eye of Hurricane Dorian was located
by an Air Force Reserve Unit Hurricane Hunter aircraft near latitude
19.2 North, longitude 65.7 West. Dorian is moving toward the
northwest near 13 mph (20 km/h), and this general motion is
expected to continue through Friday. On this track, Dorian should
move over the Atlantic well east of the southeastern and central
Bahamas on Thursday and Friday.
Looking ahead please consider these factors! (from CLR, previous
Forecaster Discussion) The expected NW to WNW turn by
strengthening Hurricane Dorian will take place Friday evening or
Saturday morning east of the Bahamas. This turn is critical. How
far west will the surface trough make it into the Gulf of Mexico?
How quickly will the subtropical ridge build to the north of
Dorian? Dorian`s eventual landfall location along the Florida east
coast will depend upon the timing and degree of WNW turn Friday
evening/Saturday morning. For now, the official track takes Dorian
into north-central Florida. The average error for the day 5
forecast point is nearly 200 miles, and Dorian is not an easy
forecast to resolve.
Upper Air/Data Collection......DR/NB
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Area Forecast Discussion
National Weather Service Phoenix AZ
430 PM MST Wed Aug 28 2019
Very hot temperatures are expected again today with portions of
south-central Arizona once again reaching excessive heat thresholds.
Yet another period of excessive heat is expected on Friday and
Saturday with most lower desert locations across seeing highs at or
above 110 degrees. Storm chances return to the higher terrain of
south-central Arizona again this afternoon and evening. Storm
chances may expand westward later in the holiday weekend with
isolated thunderstorms possibly even reaching southeast California.
Latest streamline analysis indicates the center of the
mid-tropospheric anticyclone is centered just east of Yuma, which
places much of Arizona in a weak westerly flow. Across northern
Sonora, latest satellite imagery reveals a broad area of vorticity,
which is associated with cooler temperatures aloft across
southeastern Arizona. Morning CAMs indicate convection is expected
to be favored in these areas as well as along the Mogollon Rim and
higher terrain east of Phoenix late this afternoon.
Latest trends in the HRRR, local CAMs and HREF suggest a relatively
high likelihood of an outflow boundary propagating northwestward
through the Phoenix area this evening. The boundary should be
capable of producing 30 mph gusts which will likely generate at
least patchy blowing dust across Pinal and Maricopa Counties.
Official precipitation chances across the lower deserts remain on
the high end of guidance but generally less than 20 percent, mainly
owing to a pocket of warm air aloft and weak mid-level lapse rates
across central Arizona. Latest ACARS soundings indicate MLCAPE
between 500 and 1000 J/kg, along with 100-200 J/kg of CIN.
Consequently, it will generally take increasing moisture convergence
between the terrain-driven westerly flow and a strong outflow
boundary to overcome the inhibition.
Nevertheless, a few air mass thunderstorms even within the Phoenix
metro area cannot be ruled out, given the excessive heat, seasonably
high low-level dewpoints and subsequent instability. The Excessive
Heat Warning will expire this evening. With a high temperature most
likely reaching at least 110 deg in Phoenix, this will make 28 days
at or above 110 deg, moving this year into fourth place all-time.
Over the next several days, convective activity will generally fall
on the low-grade monsoon end of the spectrum. Most of the
thunderstorms that develop will focus across the high terrain east
of Phoenix or down into southeast Arizona in areas southeast of Casa
Grande. Models continue to depict occasional inverted trofs rotating
around the high, but the high itself remains rather strong each day
and centered across the central portions of the state; this tends to
shunt the disturbances/inverted trofs to the south and they track
mainly through northwest Mexico or far southern/southeast Arizona.
On Thursday, an increase in clouds/humidity from the east plus
slight weakening of the high and 850mb temps dropping to around 29C
support high temps falling below heat warning thresholds for one day.
Guidance has been very consistent lately in calling for the
excessive heat to quickly rebound Friday into Saturday. ECMPHX
ensemble guidance from the ECMWF actually calls for higher temps in
Phoenix Fri/Sat than the currently ongoing heat event. NAEFS
percentiles for 850mb temps keeps the 90+ percentile area going
across the central deserts again on Friday and Saturday, and 850mb
temps climb to near 31C by Saturday afternoon. With the high
centered to the west of Phoenix, a drier north/northeast steering
flow sets up and 500mb heights again reach or exceed 594dm. Surface
dewpoints over the lower deserts are forecast to fall in to the
upper 40s to low 50s during the afternoon hours. Everything supports
another bout of excessive heat and the Excessive Heat Warning that
was issued earlier for much of our lower deserts will continue. This
warning includes much of southwest Arizona and far southeast
California as well as south-central Arizona and the Phoenix area.
Operational as well as ensemble guidance continues to suggest that
better chances for monsoon activity will develop Sunday into the
early part of next week. The upper high finally shifts further to
the north and east and a deeper east to southeast steering flow sets
up that is able to import healthy amounts of moisture westward and
all the way into the southern California deserts. As such POPs
Sunday into Tuesday reflect conditions climbing into the moderate
grade with 10-20 percent POPs in the deserts and chance numbers over
the high terrain. One of the better chances may be Sunday afternoon
and evening as an inverted trof looks to be moving west and around
the high, increasing storm chances over the southern part of Arizona.
Of course, as clouds and humidity rise (along with potential for
actual rain and rain-cooled outflows) high temperatures will finally
drop off to near seasonal normal levels.
.AVIATION...Updated at 2330 UTC.
South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Large forecast uncertainty into this evening is tied to maintenance
(or lack there of) of storms and outflows progressing north and west
from E/SE AZ. Moderate to high confidence exists that a better
defined boundary will progress into the Phoenix metro in the 03Z-05Z
time frame causing an abrupt wind shift some where between an
easterly and southerly direction. Lower confidence exists regarding
wind gust speeds and propagation of dust along this boundary where
lofted dust and a brief period of lower slantwise visibilities may
be the more common impact.
The lowest confidence exists with respect to the potential for TS to
be maintained or re-develop over/near terminal sites. Recent HRRR
iterations are probably most aggressive though even this output
generally only shows decaying storms. Chances of an actual TS within
a terminal footprint is probably only 10-20% at best. If storms can
form in the metro, resulting wind directions will become extremely
variable with stronger gusts potential for multiple wind shifts.
EVentually, a general easterly component should settle over
terminals beyond midnight as any activity wanes during the
Southeast California/Southwest Arizona including KIPL and KBLH:
No major aviation concerns through Thursday afternoon under clear
skies. Some measure of southerly winds will be maintained with
potential for some variability at KIPL through the evening/overnight.
Gusty winds at KBLH should decrease during the evening with less
gusty character Thursday afternoon.
Aviation Discussion not updated for amended TAFs.
Saturday through Wednesday:
The Monsoon High will migrate north of the area and towards the Four
Corners by early next week. This will result in an easterly flow,
which will be favorable for increasing relative humidities and at
least isolated storms each day, particularly across eastern Arizona.
Meanwhile, above normal temperatures are expected Saturday, though a
cooling trend is anticipated through the middle of next week.
.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.
AZ...Excessive Heat Warning until 8 PM MST this evening for AZZ534-
Excessive Heat Warning from 10 AM Friday to 8 PM MST Saturday
CA...Excessive Heat Warning from 10 AM Friday to 8 PM PDT Saturday