Mid-Latitude Cyclones to Affect Northern California Wednesday – Saturday
FEBRUARY 24, 2014
Another round of storms is on the way to northern California beginning Wednesday, however once again, these systems won’t have major impacts on our drought. Nonetheless, any precipitation we can get helps in some way, whether it be temperately moistening the ground and watering yards, to adding a few feet to reservoirs.
These systems will be the closest we’ve been to some deep mid-latitude cyclones this winter season, with Wednesday’s system dipping to between 996 – 1000mb before weakening and swinging northward. Friday’s/Saturday’s is forecast to drop to around 980 – 984mb, but it’ll remain at it’s deepest offshore before weakening and sliding south. Mid-latitude cyclones sound a bit ominous, but they’re generally the number one culprit of larger scale storms in the mid-latitudes… this will just be the closest we’ve come to some of these deeper low centers.
0z NAM’s forecast 500mb relative vorticity and MSLP valid Wednesday evening. Warm colors indicate counter-clockwise circulation.
Wednesday’s system will take the furthest north track, thus bringing us in northern California the best dynamics, and thus precipitation. This first system will have access to a decent sub-tropical moisture plume, with precipitable water values of 1 – 1.1″ making their way inland. The low itself will track east towards what would be Fort Bragg through the late afternoon, however begins to rotate northward offshore overnight Wednesday and weaken. The only reason this low rotates north is due to Friday/Saturday’s system pushing eastward into the eastern Pacific, bringing the weaker low into it’s cyclonic flow, pulling it back to the northwest and causing it to dissipate.
Before it dissipates off the Oregon/Washington coast, however, it swings a few waves of energy into northern California on the lows southeastern flank. The first wave is the initial precipitation shield, which swings into the coast early to mid-morning along the coast, and into the valley and mountains by afternoon. This first band will bring the most widespread precipitation, however another wave of energy contained in the low rotates inland during the late afternoon into the early to mid-evening. This is when things get interesting.
Models are growing increasingly bullish with the amount of instability behind the initial precipitation shield given a modest mid/upper level cold pool associated with the low. Plume of steeper mid-level lapse rates pushes overhead during the evening and overnight hours promoting surface based CAPEs of 100 – 300 j/kg in general, with smaller pockets of around 500 j/kg SBCAPE. While the low will be reenforced by a 130kt jet, most of the jet stream layer winds will be blasting southern and central California, with weaker upper level winds over northern California. However, given the low being near the coast, 250 mile pressure gradients of 5 – 8mb are forecast, bringing a band of 40 – 50kt 850mb winds, with 45kt winds extending down to 925mb Wednesday evening from the central Sacramento valley into the northern San Joaquin valley. Given the likely presence of moderate to locally heavy to even convective precipitation, enough downward energy should be available to drive down some 30 – 45mph wind gusts from around Yuba City to Stockton.
0z NAM’s forecast 850mb winds and MLSP early Wednesday evening.
Convective potential appears highest from late evening Wednesday through early Thursday afternoon, peaking in the pre-dawn hours (2 – 5am). Given a decent (though not extreme) surface low off the CA/OR coast, this promotes veering winds in the mid-levels that promote rather decent directional shear, with 0 – 3km storm relative helicities of 100 – 250 M2/S2, and notable low-level shear (0 – 1km bulk shear) of 20 – 35kts… hodographs are clockwise curved and hooking, indicative of an environment supportive of supercellular structures. Since this is a nocturnal setup, instability is a bit elevated… above 900mb it appears. Given the potential for slightly-elevated convection, I believe tornadic potential will be limited. However, robust updrafts could support some hail… possibly near severe levels if enough instability is present.
0z NAM’s forecast ML (mixed-layer) CAPE valid for around 4am Thursday morning.
0z NAM’s forecast 0 – 3km storm relative helicity valid early Thursday morning (4am-ish again), indicating a wide swath of 100 M2/S2+ values (yellow).
Diurnal heating combined with another weak rotating inland before the low rotates too far north to affect northern California. This last wave will bring another increase in showers during the day Thursday before clearing out by evening.
Snow levels with the Wednesday/Thursday system primarily start out around 6000 – 7500ft Wednesday afternoon and evening, but lower quickly overnight into Thursday to around 4000ft as cooler, more unstable air moves overhead. 1 – 2ft of snow appear likely above 6000ft, with 3 – 9″ above 4000ft.
QPF wise, .5 – 1″ of rain is possible in the lower elevations including the coast and valley, with 1 – 2″ in the foothills and 2 – 3″ of liquid in the mountains (see winter precipitation in previous paragraph). Locally higher and lower QPFs are possible due to what could be a little or a lot of convection.
Next system pushes toward the coast Thursday night, with precipitation rotating northward into northern California as early as the pre-dawn hours of Friday. Deep subtropical moisture plume will remain confined to southern California with this system, however strong lift will still wring out moderate precipitation. Models are still fluttering with the track of the second low, hovering between the central coast and southern coast… but as long as impact is south of SFO, the best dynamics will be directed at southern California. Even then, we aren’t in too bad a position ourselves here in northern California, and we shouldn’t do too bad precipitation wise.
It appears another pocket of instability will move overhead the Sacramento/San Joaquin valley Friday… but I find it too early to call out convective potential just yet, as models have been all over the place with how exactly this system sets up.
It appears snow levels with the Friday/Saturday system generally stick around 5000ft, but could flutter up or down by a thousand feet depending on the track of the low.
It appears another .75 – 1.25″ of rain could fall across the valley Friday into Saturday, with 1 – 3″ in the mountains (falling as another 1 – 3ft of snow above 5000ft). The initial front Friday could bring some gusty winds given a 980 – 984mb low offshore, with 850mb winds in the 40 – 50kt range, I wouldn’t rule out another band of 30 – 45mph gusts Friday morning from Yuba City southward.
We should start to dry out Saturday night into Sunday as the trough ejects east, however the GFS wants to keep some leftover instability for some showers across the area Sunday. GEM/ECMWF try to indicate another system early next week, but confidence is very low on that system right now.
I’ll have another update out Wednesday specifically regarding the convective threat overnight into Thursday, and another update Thursday on the Friday – Saturday system.
Widespread Thunderstorm Threat Wednesday & Thursday
FEBRUARY 25, 2014
A very dynamically strong, subtropical moisture plume supported system off the California coast will rotate northward along the northern coast of the state through the day and overnight Wednesday, putting northern California in the perfect region for the system’s best available large-scale accent, moisture, and with this system, especially instability.
Latest models now indicating precipitation onset a bit sooner… perhaps by mid to late morning as some pre-frontal moisture combined with increasing lift combine to develop the first round of showers. This seems possible given cooling cloud tops noted on GOES-west IR imagery are noted just 200 – 300 miles offshore.
Behind the first round of showers, the cold frontal precipitation band is expected to rotate into the coast by mid to late afternoon, and deeper inland through the evening. As the surface low nears closer to the coast during the evening, a band of strong low to mid-level winds pushes overhead along with the cold front, which combined with very strong lift and decent moisture should promote heavy precipitation, which will likely drive down some 35 – 45mph surface wind gusts across the valley as a belt of 40 – 50kt 850mb winds slides through.
0z 12km NAM’s forecast 850mb winds for around 10pm Wednesday evening.
In the mid/upper levels, -20 to -25c 500mb temps associated with the low slide overhead behind the cold front, which combined with surface temperatures in the mid-50s across the lower elevations due to decent low-level moisture return, promotes a plume of relatively steep mid-level lapse rates which pushes overhead just ahead of and especially behind the cold front through the night Wednesday into Thursday.
Decent lapse rates along with surface dewpoints in the 50s promote SBCAPE values in the 200 – 500 j/kg range in general from the coast inland into the Sacramento and northern San Joaquin valley overnight into Thursday, and lifted indexes between -1 and -4c. This amount of instability combined with large amounts of lift on the southeastern flank of the surface low provides a prime environment for thunderstorm development; both along the cold front, and behind it from Wednesday night through Thursday afternoon.
Forecast mixed-layer CAPE and 10-meter winds for around 10am Thursday plotted through the 0z 12km NAM.
Forecast sounding with surface based parcel highlighted. for the southern Sacramento valley for around 10am Thursday.
Along the cold front, while instability isn’t as strong as what lies behind it, very strong lift and a decent low-level jet should prove favorable for at least somewhat in the way of embedded convection. If colder air aloft pushes in somewhat faster aloft over the cold front, a large-scale line of thunderstorms could embed themselves within the frontal precipitation band. Storms such as these would be capable of driving down even stronger winds than what you’d see in stratiform precipitation… possibly near severe criteria if such an event were to occur given the strong mid-level winds.
Behind the front, moisture and lift still exists, nearly as strong as the cold front, which will continue to support waves of showers and thunderstorms through the night. Low-level shear is strongest with the band of strong mid-level winds that accompanies the cold front, but even behind the front 850mb winds remain elevated – in the 20 – 40kt range, with winds increasing in strength with height. Given there being a 992 – 996mb low off the northern CA/OR coast Wednesday night into Thursday, winds will certainly veer with height… with surfave winds from the south to southeast in the valley and winds from 850mb to 500mb being out of the west-southwest to dew west by Thursday morning. This notable veering creates some helicity (essentially horizontal rotation in the atmosphere, or a “rolling” motion), with 0 – 3km storm relative helicity values in the 100 – 250 M2/S2 range Wednesday night into the early afternoon Thursday. These shear parameters combined with the instability forecast would technically make for a decent environment for rotating thunderstorms, however a lack of surface heating during the nighttime hours may inhibit stronger updrafts, thus limiting vertical rotation potential.
0z 12km NAM’s forecast 0 – 3km SRH valid for around 10am Thursday. Orange and red indicates 150 – 200 M2/S2 values, which support some notable hodograph clockwise curves.
0z 12km NAM’s forecast hodograph for the southern Sacramento valley. In coordination with above 0 – 3km SRH values, there is indeed a clockwise hooking feature.
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