National Weather Service Forecast Discussion

Dicussion provided by the National Weather Service Forecast Office located in San Diego, California

FXUS66 KSGX 181005

Area Forecast Discussion
National Weather Service San Diego CA
305 AM PDT Mon Mar 18 2019

Fair and warm today, with a gradual cooling trend through midweek.
Some low clouds and fog could develop along the coast, mainly Monday
night. A Pacific trough will arrive Wednesday morning with showers
and gusty southerly winds. A few thunderstorms are possible as well.
Snow will fall in the mountains, and could become heavy at times
Wednesday night through Thursday. A much weaker system could bring a
few showers again by the weekend, and even next week the potential
for rain and mountain snow continues with temperatures below average.



Skies were mostly clear at 2 AM PDT. Only a few clouds were evident
in satellite imagery over the coastal waters. The sfc pressure
gradients from the deserts remained offshore but were weakening and
trending more onshore to the east. Wind reports were light. Better
onshore gradients will mean cooler and more moist marine air will
gradually work inland through Tuesday for cooler days and more
coastal marine clouds. Still warm today well inland, but the sea
breeze will make it cooler near the Coast.

A weak ridge over the SW today, will give way to an approaching
trough over the far EastPac by Wednesday. The trough will have
several spokes of energy rotating through it. The first arrives Wed
morning with a negative tilt and difluence aloft as the nose of a 90
KT, 250 MB jet drives east across far northern Baja. This puts us in
the favorable left front quadrant for lift and possible heavy
convective showers or thunderstorms. Precipitable water will be
increasing to just under one inch, and the 00Z NAM12 has a fair
amount of MU Cape. These parameters all point to at least the
possibility of some locally heavy downpours on Wednesday.

The operational models all show an upper low closing off on
Wednesday, but it is handled differently by each. Heights fall over
SoCal through the day as colder air feeds into the trough and the
upper low drops SE across CA. All of the recent ECMWF runs have
consistently been deeper and slightly farther West with the trailing
shortwave and upper low. Probably due to a slower demise of the
massive ridge aloft, centered over SW Canada. This slower trend
makes sense and may give the ECMWF model an advantage in forecasting
the ultimate depth and track of the incipient closed low to affect
us midweek.

Following the ECMWF, the colder and deeper solution means showers of
significance will likley continue through Thursday, with snow levels
a bit lower than forecast by N.A. blends. With that in mind, snow
amounts still look modest below 6500 FT, but at or above 6500 FT,
six inches or more is possible from Wednesday afternoon through
Thursday night. Convection could enhance amounts locally. Storm
total rainfall amounts will be light to moderate overall (0.25-0.50
inch) coast/valleys, but there will likley be heavier showers which
could cause local impact due to runoff and ponding of water,
particularly in urban areas and burn scars. For the deserts,
rainfall still looks minimal (0.05 to 0.20 inch). Maximum coastal
slope and thunderstorm amounts could be around an inch or more

In terms of wind...look for some gusty southerly winds 15 to 25 mph
coast and western valleys Wednesday morning, becoming westerly and
gusty Wednesday PM through Thursday (mainly mts/deserts).

Friday still looks mostly dry ahead of another fast-moving Pacific
trough. This one does not look to amplify, and models show it
passing to the north of us with minor impact here. Gusty westerly
winds are expected over the mts/deserts, and there is a chance for
some light showers along and west of the mts on Saturday. Once again
the ECMWF is a bit stronger with the wave.

The global models show potential for a stronger storm system again
next week, but timing and track differences are too large at this
point for any forecast confidence of impact over SoCal.


180900Z...Mostly clear skies today and tonight, but with isolated
coastal low clouds based at 500-800 feet MSL and local vis below 3SM
remotely possible through 16Z. Low confidence of impact at coastal
airports. Slightly greater coastal cloud coverage is expected
tonight after 05Z.


A weak Pacific storm will likely bring showers with a slight chance
of thunderstorms, along with an increase in winds and seas Wednesday
through Thursday. However, winds and seas do not appear hazardous.


Building swell this week will generate surf up to 4-8 feet Wednesday
into Thursday. This would bring a high rip current risk before surf
subsides on Friday.





NWS SGX Office Area Forecast Discussion