Winter-Like Storm Friday

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11:15am (Friday) Update:

Shortwave offshore has taken a more easterly track than earlier modeled, and is supporting early development of convection as the favorable southerly low-level flow ahead of the offshore shortwave/vortmax increases into the afternoon. Widespread clearing has not occurred due to a new band of clouds that rapidly enhanced through the morning ahead of the vortmax, however surface temperatures in the mid to upper 50s in the valley combined with dewpoints in the upper 40s to low 50s from the central Sacramento to northern San Joaquin valleys has promoted SBCAPE values already in the 100 – 300 j/kg range. Visible satellite imagery indicate rather widespread cloud cover to the coast, however the band of clouds that rapidly enhanced this morning has stopped most enhancement progression and is spreading east and beginning to decay a bit. This may allow some scattered breaks in the clouds through early afternoon, however forcing and lift is increasing as the trough becomes strengthened by a strong jet streak slamming and rotating around the base of the trough. Increasing lift and forcing is already allowing new convection to fire up across northern California late this morning along the east side of the northern San Joaquin to central Sacramento valley, with more robust storms firing in Tehama county.

Visible satellite, radar, and 10-meter wind directions from 11:15am. Some key features are the offshore convective band associated with the vortmax and some decent storms in Tehama county.

Visible satellite, radar, and 10-meter wind directions from 11:15am. Some key features are the offshore convective band associated with the vortmax and some decent storms in Tehama county.

Between convective bands, any further surface heating will promote boosted instability with SB/MUCAPEs perhaps rising to around 500 – 600 j/kg through late afternoon. Shear profiles are not great for widespread robust convection, however terrain enhanced low-level backing of winds does create some very slight clockwise curved hodographs due to weak winds aloft, given mean jet-layer wind field is now in central and southern California. Where hodographs are not slightly curved, they’re more straight-line with somewhat veer-back-veer profiles favoring bands and clusters of storms rather than discrete storms. Thus, overall threat from storms will likely be hail given pool of 7 – 7.5c mid-level lapse rates sliding overhead this afternoon and perhaps gusty winds… however any discrete storms will still pose a non-zero tornado threat.

Forecast RAP sounding near Sacramento for mid-afternoon, indicating decent instability, but shear lacking in regards to rotating storms.

Forecast RAP sounding near Sacramento for mid-afternoon, indicating decent instability, but shear lacking in regards to rotating storms.

Thursday evening update:

A potent shortwave embedded in a trough offshore will make for an active Friday in many ways, including low-elevation rain, mountain snow, and potential thunderstorms in the valley.

This trough is backed by a rather strong 140 – 160kt jet streak entering the back side of the trough’s jet ahead of a strong upper-low over the Allusion Islands, and at the same time has tapped into a plume of precipitable water (PW, PWAT) plume of .7 – .9″, which will sag south through the overnight hours int Friday morning as the trough’s cold front slides south through northern California during the early morning hours Friday (generally between 3 – 7am). This front for the most part will be confined to the mountains, however some valley showers are very possible.

0z GFS's forecast 300mb winds and 500mb heights valid Friday afternoon, indicating a strong to very strong jet streak stretching from the eastern Pacific far to the west.

0z GFS’s forecast 300mb winds and 500mb heights valid Friday afternoon, indicating a strong to very strong jet streak stretching from the eastern Pacific far to the west.

By mid to late-morning, it seems a bit of a lull/dry slot between waves within the trough may occur, giving the valley a chance to perhaps loose some of the stratus cover from the overnight front. If high resolution models are correct, enough clearing may occur between the cold front and offshore vortmax/upper low to allow surface temperatures to skyrocket into the upper-50s to mid-60s, which would equate to extremely steep low-level lapse rates. 500mb temperatures aloft associated with the trough’s cold pool are progged around -29 to -32c, which is quite cold for this time of year. Not only does this mean snow levels in the 4000 – 5000 foot range, this also means some steep mid-level lapse rates given intense difference in low-level temperatures and upper-level temps (probably around 7 – 7.5c).

0z NAM's forecast 500mb relative vorticity, indicating a stream of high positive vorticity associated with a vort-max headed toward the north-central to central coast of California, aiding upper-level dynamics.

0z NAM’s forecast 500mb relative vorticity, indicating a stream of high positive vorticity associated with a vort-max headed toward the north-central to central coast of California, aiding upper-level dynamics.

Dew points in the mid to locally upper 40s Friday afternoon and highs potentially in the 60s would equate to SBCAPEs in the 200 – 400 j/kg range, with some pockets around 500 – 600 j/kg where overnight/morning rain boosts dews. This is sufficient instability for thunderstorms to develop in, and given those potentially relatively warm surface temperatures, upward motion of surface energy (warm air) would be fairly strong.

0z 4km NAM's forecast MLCAPE, valid around 5pm Friday, indicating a wide swath of 100 - 500 j/kg values across northern California.

0z 4km NAM’s forecast MLCAPE, valid around 5pm Friday, indicating a wide swath of 100 – 500 j/kg values across northern California.

Shear is still a bit of an if with this system. Some model soundings indicate some veer-back-veer profiles; not favorable for tornadic storms. In other cases, low-level winds out of the south from the delta combined with a westerly mid/upper-level component from the jet aloft create better shear/wind profiles, though winds themselves appear weak due to the bulk of jet-level winds being directed into central and southern California.  LCL heights in the 500 – 1000m range indicate relatively low-based storms, which would be decent for rotating thunderstorms, basically leaving shear as the only potential downside to the system.

Forecast sounding from the 0z NAM late Friday afternoon.

Forecast sounding from the 0z NAM late Friday afternoon. In the low-levels, you can note some southerly winds, and in the mid-levels, some westerlies (between 850 and 600mb or so)… which is good. However, once you get to around 500mb, winds turn SW again, which would effectively weaken or limit rotation (both horizontally and vertically). If the shortwave moves slightly slower, vertical wind profiles would be more favorable for rotating storms.

One thing that may locally boost shear is the vortmax/upper low that is expected to travel right along the coast and potentially impact somewhere around SFO. This vortmax may bring with it a belt of somewhat stronger winds in the mid/upper-levels, which would help boost shear a bit depending on exact impact location.

Based on current data, it appears thunderstorms Friday would be most capable of hail and perhaps strong winds due to a bit of a dry layer visible in forecast sounding not too far above the surface. Depending on convective coverage, more discrete storms could be capable of a tornado threat, but given the strange shear, confidence is low. Best threat of any thunderstorms appears to generally be confined to the Sacramento valley due to better heating potential during the day, with the cold front still tracking through the San Joaquin valley during the morning. I’ll probably post a quick update to the top of this post late in the morning or in the early afternoon Friday as the setup commences.

April 24th Map

April 24th Setup

April 24th Threats

Showers could linger through Friday evening as the vortmax and base of the trough push eastward, before the exit jet slides overhead Saturday to clear us out. Precipitation amounts look to range from none to .3″ in the valley, to .25 – 1″ in the mountains and .25 – .5″ along the coast. Higher amounts will of course be possible in thunderstorm/strong convective shower tracks. Snow levels down to around 4000 – 5000ft will allow 3 – 6″ of snow above 4500 – 5000ft, with 5 – 12″ in the highest of elevations of the sierra… perhaps locally a bit more.

A weak and decaying front is then forecast to slide through northern California Sunday morning, likely clearing out by afternoon. Next week looks pretty warm as strong ridging builds in, with the valley’s first widespread 90s looking possible by mid or late-week, or at least widespread mid to upper-80s.

Stay tuned for Friday’s quick update regarding thunderstorm potential if you have a convective addition as much as I do.

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