Wednesday/Thursday Storm Update: Rain, Snow, & Thunderstorms

Wednesday/Thursday Storm Update: Rain, Snow, & Thunderstorms

Posted January 28, 2014 by wxtracker15 & filed under Active, Convection, Storm,Thunderstorms, Trough, Winter.

After weeks upon weeks of unseasonably dry and warm weather across the state, over the last few days a decent sized system has been taking shape in the east-central Pacific, north of Hawaii. This system consists of a couple of upper lows spiraling around north of the fierce 100 – 130kt jet within the trough, and a large plume of sub-tropical moisture steaming on up from the Hawaiian islands area.

Over the last couple of days this stream of moisture has been hosing Oregon and far northern California with some light to moderate rain, which itself isn’t very significant, but the depth and amount of moisture present is notable, with precipitable water values in excess of 1″, and may reach values of around 1.3″ Wednesday ahead of the main cold front involved with this system.

As the upper low(s) begin their eastward track Wednesday, a tightening in thermal/temperature gradients develops in the eastern Pacific, a new low is modeled to develop and strengthen lift to the east of the low, stretching east into northern California along a stationary/warm front. Along this boundary is where precipitation will begin to develop and fill in through the day Wednesday. Precipitation will likely fist start along the coast as the stream of moisture begins to drop south out of southern Oregon, then in the mountains as orographics strengthen. Still some uncertainty as to when precipitation may begin to fill in-in the valley, but generally models indicate low-level saturation and strengthening in lift enough for precipitation development in the lower elevations by mid to late afternoon, perhaps as early as early afternoon in some locations and as late as early evening in other locations. In the San Joaquin valley, precipitation may not begin until mid to late evening due to the front not speeding up it’s southward run until around that time.

00z 4km NAM’s forecast reflectivity valid for the later evening Thursday.

As the system strengthens just offshore Wednesday evening, frontal and pre-frontal lift also increases in intensity across northern California through the evening Wednesday, also increasing precipitation intensity. Given 1.2 – 1.3″ PW values and 700mb vertical velocities of 40 – 50 UB/s in the valley and 700mb VV’s in the 50 – 90 UB/s range in the foothills and mountains ahead of the cold front Wednesday night into early Thursday morning, moderate to heavy precipitation seems quite likely up until frontal passage Thursday morning.

4km NAM’s forecast 700mb vertical velocity and 700mb heights valid for mid-evening Wednesday.

Snow levels are forecast to start out rather high given the warm origins of the system’s moisture tap, likely starting out up around 7500 – 8500ft in most sierra locations. Through the night Wednesday into Thursday morning, snow levels steadily drop off to perhaps around 4000 – 5000ft Thursday. Even after frontal passage Thursday morning, upslope flow and orographic effects will continue to support precipitation development over the higher elevations.

4km NAM’s forecast precipitation type valid during Wednesday morning, indicating little snow in the sierra due to high snow levels.

4km NAM’s forecast precipitation type for early Thursday morning as the initial cold front passes. Snow level drops off a few thousand feet compared to the evening Wednesday.

Will the valley dry out after the first front passes early Thursday morning you ask? Well, it appears it may not – in fact, it could be quite the contrary. High resolution models are indicating that a secondary front may develop in the colder air behind the sub-tropically infused first cold front, and slide through northern California later Thursday morning into the mid or even late-afternoon, just in time for peak heating.

00z NAM’s forecast reflectivity valid early Thursday afternoon, indicating another round of precipitation moving inland with a shortwave.

This second front/precipitation band appears to be associated with the main shortwave that drove in the initial cold front, which may energize cloud tops enough with the second band to support some convection along the second band. If the valley were to see any cloud breaks Thursday morning before things cloud back over ahead of the secondary system, instability could be enough to assist in the development of some thunderstorms Thursday afternoon into the early evening in the valley and foothills… perhaps even along the coast if the shortwave is strong enough.

Even with limited sunshine, high resolution models are generating surface based CAPEs of 100 – 300 j/kg, with more bullish solutions, such as that of the 4km NAM’s, indicate values of around 500 j/kg along a band from Yuba City to Modesto. In addition to a bit of CAPE, lifted indexes (LI) are also forecast to dip to around 0 to -3c depending just how unstable the valley is able to get. Now, if I’m talking convection here, I just have to dab into the shear aspect of this setup (then again, who wouldn’t?). Taking a peak at the NAM’s hodographs, I can find my favorite shape present in most Sacramento valley site forecast hodo’s, and that’s a clockwise curve. What does this clockwise curve mean you ask? Well, it indicates that there is some decent turning going on with height wind wise, which as some of you may know is great for developing rotating updrafts in thunderstorms, and thus, supercell structured storms. To support these hodographs indications, 0 – 3km storm relative helicity values are progged in the 100 – 200 M2/S2 range, which is well within the range of which mini-supercells are known to have occurred/developed in. The only iffy thing about this event is coverage of showers/storms Thursday, and when the second front exits the valley.

4km NAM’s forecast surface based CAPE valid for later Thursday afternoon, indicating 250 – 500 j/kg values in the southern Sacramento/northern SJ valley.

NAM’s forecast hodograph for Thursday afternoon.

If the front exits the valley by or before around 1 – 3pm, there would be a few hours left of sunshine in the valley to increase instability and lapse rates, perhaps enough to get storms developing and possibly rotating if instability were to be strong enough. Another, well, option per-say would be for there to be some sunshine ahead of the second front, which is also possible, but not set in stone, as there could be lingering status over the region. In either of these solutions thunderstorms would be possible, either along/within the secondary precipitation band, or behind it if there is enough time left before sunset.

In the case thunderstorms do develop, as mentioned before, there is enough directional shear for some rotating thunderstorms/supercellular structures. And if storms of such strength were to develop, they could become capable of hail, gusty winds, and weak tornadoes. Again, that’s if there is enough instability and shear for mini-supercells to develop and sustain themselves, which is possible, but not set in stone.

Once the trough’s base shifts east, lift ceases to exist and precipitation also ceases to exist through the night Thursday. Throughout this ~48 hour event, .25 – .50″ of rain appears possible for a good chunk of the Sacramento and northern San Joaquin valley, with amounts up to an inch possible along the east side of the valley, where uplsoping/precipitation banking along the foothills and western slope stack up and intensity precip from the east side of the valley up into the foothills and western slope of the sierra. Along south and southwest facing slopes, where orographics are maximized from this system, 3 – 5″ of precipitation appears possible, with 2 – 3″ possible in most of the west slope as well. .75 – 2″ also appears likely in the coastal mountains, with 2 – 5″ along the northwestern corner of the state. It appears the west side of the valley could be moderately to severely rain-shadowed, limiting precipitation quite a bit compared to areas that aren’t directly adjacent to mountain ranges.

Snow wise, currently looks like around 8 – 10″ is possible around 4500ft, with 1 – 2ft above 6000ft. These amounts will likely be enough to cause some travel issues across the sierra Wednesday night and Thursday, and while winds won’t be all that significant, snowfall intensity at times could be heavy enough to limit visibility quite a bit, adding to the travel danger of driving on snow.

Things are forecast to remain dry through the weekend into at least early next week as a ridge takes over once again, but we won’t be climbing into the 70s any time soon, as we’ll be stuck under some much colder air draining southward from Canada.

If anything changes forecast/model wise Wednesday, especially regarding the threat of convection Thursday, I’ll be sure to update this post or post an entirely new post if needed. Stay tuned, and be sure to check out the site’s Facebook page (can be found to the right) for “nowcasts” and up to the minute updates on active weather.