Scientific Forecaster Discussion
fxus65 ktwc 191750
Area forecast discussion
National Weather Service Tucson Arizona
1050 am MST Tue Nov 19 2019
Synopsis...a Pacific storm system will bring a good chance of
showers and thunderstorms spreading in from the south by this
afternoon and evening. Some storms will have heavy rain and strong
winds. A chance of showers will continue Wednesday before a
stronger and colder system brings a good chance of valley rain and
mountain snow Wednesday night into Friday morning. Significant
snowfall expected above 7000 feet. Temperatures will cool to below
average by the end of the week, dropping 25 degrees by Thursday.
Discussion...no major changes to the overall forecast this
morning. Evolution of the convection and storm system remains on
track as there is excellent agreement in high resolution forecast
models. Due to the active weather and in collaboration with dm,
decided to hoist an 18z balloon. Early indications from the
sounding are a significant increase in moisture with the morning
mid level cap rapidly eroding. The wind regime is also increasing
in the mid and upper levels. These changes will help set the
stage for today's busy weather.
Activity has already crossed the border with a couple
embedded strong thunderstorms in western Pima County. These storms
are likely producing very heavy rain given the robust kdp values
from Yuma dual pol radar. Expect this activity to gradually push
north and strengthen/expand in coverage as it approaches the
Tucson Metro around 20z. The best chance for strong to perhaps
severe thunderstorms and locally heavy rain in the Metro will be
from 20z this afternoon through 04z this evening. Currently the
hrrr/rap/href indicates potential for training convection, which
could cause localized flash flooding. This will be watched closely
as convection moves north.
After this first round of morning/early afternoon convection,
expect another round of strong to perhaps severe thunderstorms as
cape values along and south of the border are in the 500-1500 j/kg
range. This instability will build and expand north this
afternoon. This will be in conjunction with a robust increase in
mid to upper level flow. This is when shear will increase to
30-50kts, which will enhance thunderstorm potential and provide a
chance for convective organization or even a transient low top
supercell. In addition, href indicated precipitable waters will likely rise to
1.2" to 1.3" this afternoon. It is shaping up to be quite the
Another updated afd will be issued later this afternoon.
For more information about kemx radar....please see statement
Aviation...valid through 20/18z. Increasing clouds, -shra
expected along the international border starting around 19/16z and
moving north becoming widespread over Arizona for the rest of the day
and evening. Chance of thunderstorms and rain as well. Brief MVFR for ceilings/visibility
possible in shra/tsra. Surface winds will remain light and variable
until 19/16z, then surface wind southerly 8-12 kts gusting up to 18kts area
wide. Gusty outflow winds possible from thunderstorms and rain. Aviation discussion
not updated for taf amendments.
Fire weather...widespread showers expected to cross the
international border later this morning and move north over most
of Arizona and will persist until Friday. Slight chance /chance of
thunderstorms as well. For the higher elevations, precipitation
will begin as rain but transition to snow Wednesday evening as
snow levels drop. With the expectation that lingering valley rain
and mountains snow showers will continue Friday, significant
snowfall accumulations are possible above 7500 feet by the the
time the storm exits. Conditions improve Saturday and Sunday with
dry weather returning.
South to southwest 20-ft winds of 15-20 mph with higher gusts are
expected Wednesday and Thursday, however relative humidity values will remain above
35 percent. Otherwise, 20-ft winds will be terrain-driven at less
than 15 mph. Above normal temperatures continue through Tuesday,
then drop to below normal levels Wednesday into the weekend.
Previous discussion.../issued 331 am MST Tue Nov 19 2019/ the low
just off the central Baja Coast now knows that the low digging
southward from the Pacific northwest is there, and is reacting
accordingly as it lifts northeastward toward southern Arizona
today. Just ahead of this, we have seen an unseasonably deep
moisture increase pushing in from the south with residual moisture
fields from Raymond remnants wrapping into the low. A good shear
and strong cape environment is developing on the northeast
quadrant of the low and shifting into southern Arizona today.
Moisture will deepen to near record levels for the season with up
to 1.2 inches precipitable water pushing in from the south by mid
day. This will all combine into a strong thunderstorm potential by
this afternoon with strong winds and heavy rain a concern.
Note: the kemx radar will be out of commission through Thursday or
Friday due to an upgrade that had to be scheduled well in advance.
Through this period, GOES-west mds sector 1 will be centered over
Arizona for 1 minute satellite data beginning 11 am MST.
Expect something of a lull between systems as the first opens up
and lifts northeast of the area Wednesday, and the stronger colder
system digs into southern Nevada and Southern California. Snow
levels will begin to drop Thursday morning, hitting about 8k feet
after midnight and 7200 ft around sunrise. Expect a drop toward
6500 feet behind the front Thursday afternoon. Considering the
excessive moisture fields that this stronger and colder low will
still have to work with, we could be talking about some serious
snow fall above 7000 or 7500 feet Thursday and Thursday night. I
wouldn't be surprised if we ended up with some storm totals over
18 inches above 7500 feet (especially Mt Graham and southern white
mtns). A Winter Storm Watch has been issued for all elevations
above 7000 feet Wednesday night into Thursday night. Watch for
strong gusty winds at times Wednesday and Thursday due to the
enhanced surface gradient.
Afternoon highs in the 80s again yesterday, but with the moisture
increase we should knock several degrees off today. By Thursday
or Friday we'll be struggling to climb out of the 50s. Partial
clearing from the west Friday as the low lifts quickly toward the
Central Plains. Weak troughiness in its wake will keep
temperatures from jumping back up very fast, with daytime highs
still in the 60s around Tucson next weekend.
We'll see if another Pacific low can bring more weather early next
Flash Flood Watch through Wednesday evening for azz501>515.
Winter Storm Watch from Wednesday evening through Thursday
evening for azz510>514.