Tag: Chip Redmond

November is typically characterized by increases in cold air intrusions across much of the United States. This also begins what is considered the dry season for much of the Plains and portions of the inter-mountain west. Rainfall begins to become more prevalent further south along the Gulf and where Gulf flow (southerly winds carrying moisture from the Gulf into the US) can become established before a storm system. This flow typically becomes established ahead of an advancing cold front, but is pushed further east than during the summer months. Multiple cold fronts with very little moisture/flow recovery time between can greatly influence fire activity for the Mississippi Valley and eastward. Wide sweeping cold fronts are common and typically drier as moisture becomes more limited, dominated by Canadian/Arctic air masses.

Vegetation is now dormant across most of the continental US and foliage/grasses can cure easily. The Appalachians have a vigorous deciduous tree canopy and provide new debris and additional uncompacted litter for increased fire occurrence/sustainability. Strong cold fronts tend to bring much drier air and windy conditions that only fuel fire conditions along these slopes and further to the east/southeast. The fuel load depends greatly on the water received from tropical systems during the late summer/early fall. If there is an active tropical season, fuel loads can be greatly enhanced. Therefore, the region is greatly dependent on the tropical/drought status of the area and their subsequent influences on the fire season.

Overnight relative humidity recovery is typically much higher across the US than the summer. This is due in part to the lack of solar radiation. Less sunlight reaches the earth’s surface in the Northern Hemisphere during winter. This in turn prevents the surface from warming, allowing the surface temperature to drop during the overnight, approaching saturation much easier, and providing increased humidity. Therefore, often fires in late fall/early winter may make a big run in the afternoon when the surface is warm, but then easily/quickly lay down once the sun sets. There are some exceptions including strong storm systems and terrain influences, but typically this is the case.

Complicated weather pattern currently across the US. A large high pressure sits over the inter-mountain West, an elongated front extends from Texas northeast into the Great Lakes, and another cold front is sweeping south into the northern Plains.

Fire weather focus is for southwestern California, where breezy off-shore winds continue Wed/Thurs in response to diverging winds of the high pressure centered over northern Utah. Sustained winds along higher terrain are expected up to 20-25mph with gusts to 45-50 (see graphic below). With these downsloping winds, very dry/warm conditions are expected with RHs in the mid-to-upper single digits. In combination with ongoing drought conditions, this will create explosive fire weather conditions Wed afternoon and Red Flag Warnings are posted. Winds are expected to weaken Thurs as high pressure moves northward. However, some areas will still see aggravated fire weather conditions with gusts exceeding 20mph in southern CA. After Thurs, winds will be much calmer for the remainder of the period but conditions will remain very dry. Daytime RH isn’t expected to exceed 15% for the entire period with very poor overnight recoveries into the weekend.

Elsewhere, several cold fronts are expected to traverse the Plains and Eastern US through the period. With these frontal systems, quick periods of dry and/or windy conditions are possible. The first front is currently entering the northern Plains and will bring a period of windy conditions for central South Dakota with RH dropping to 20-25% Wed afternoon. Overnight Wed the front moves further southeast into the Mississippi Valley, reaching the Appalachians Thurs. Friday, it reaches the southeastern US and windy conditions move offshore. However, enhanced fire concerns exist with RHs ranging 20-25% from east Texas to northern Florida. Friday afternoon, another quick moving cold front enters the Plains with strong winds across the northern Plains. This second front will move eastward and repeats the process through Sunday.

A third, much stronger cold front moves south from Canada on Sunday evening. This front will bring much colder air to much of the central and eastern US early week. Very strong winds are possible with this front on Sunday/Monday in the central Plains both ahead and behind the cold front. Enhanced fire conditions are possible across the central Plains through this 48 hour period, especially for portions of eastern Colorado, Kansas, Nebraska, and Oklahoma.

Some rainfall is likely from these fronts, however, much of it will avoid the southeastern, central, and southwestern US. Image from NWS Weather Prediction Center – 5 day rain predicted rainfall total.

Weather Highlights:

Southern CA: Red Flag Warnings for Wed, critical fire weather with strong winds (gusting to near 50mph) and very low RH (4-9%). Winds will weaken Thurs (gusting to 20mph), and become calm Fri-remaining very dry. Poor overnight RH recovery through the week, and into the weekend.

ND/SD: Enhanced fire weather conditions on Wed & Fri afternoon with strong northwest winds (gusting to 60mph) and low RH (20-25%).

TX/LA/MS/AL/GA/FL: Locally enhanced fire weather conditions on Fri-Sun afternoons with low RHs (25-35%).

Western KS, NE, OK/Eastern CO, WY: Beginning Sun morning and continuing until a cold front passage Monday afternoon, very windy conditions (gusting to 55mph) and low RHs (5-15% increasing Mon afternoon).

Chip

Yet another upper level trough providing some active fire weather conditions today across northern CA/southern OR and most of WY/western SD. With this trough, a lack of mid-level moisture is preventing any thunderstorm activity across the northwestern US. However, very dry conditions (aggravated by drought in CA/OR) combined with windy conditions are developing critical fire weather conditions in both these regions. A surface cold front associated with the trough will slide southeast across the northern Rockies Wed evening and into Thurs. This front will increase humidity across WY/SD lessening fire danger and shifting winds to the north for Thurs.

The same cold front will push south/west across OR/CA overnight Wed. This front will continue mixing overnight with breezy winds and poor RH recovery-continuing elevated fire danger through Wed night. Through late week and into the early weekend, easterly winds will provide additional downsloping off the Rockies and continue increased fire risk with poor overnight RH recovery through Sun. With an off-shore trough developing late-week and weak ridging over the Rockies, temperatures will increase to 90F+ across much of CA/OR/WA. Very dry conditions and RHs at/below 20% will expand eastward/northward across the region and into the central Rockies through the weekend. Breezy easterly winds will continue through Fri for much of OR/northern CA/NV and gradually diminish overnight Fri. As the off-shore upper level trough approaches the West Coast Sun/Mon, westerly winds will increase for much of the northern CA/OR/WA/ID, which may develop widespread critical fire weather conditions. No precipitation is expected for northern CA/NV and WA/OR/ID through the period.

The other major weather highlight will be soon-to-be Hurricane Norbert near the California Peninsula. Current forecast model trends depict moisture streaming northward into the southwestern US by late weekend. Developing monsoonal conditions across AZ/NM/UT/CO late week will provide chances of scattered thunderstorms. However, much more widespread rains/storms could develop for portions of southern CA/NV and much of AZ/UT/CO with the influence of Norbert. Great uncertainty still exists on the exact track of this system, though confidence is increasing on widespread heavy rains late weekend/early next week.

Weather Highlights:

WY/western SD/northern CO/NE Panhandle: Critical fire weather Wed with breezy westerly winds, RH’s <20%, and warm conditions. Cold front crossing region tonight increasing RH and winds increasing and turning northerly post-front. Becoming very dry/warm Sun/Mon with increasing fire weather threat.

OR/northern CA/northern NV: Critical fire weather Wed through Thurs with breezy northerly wind, turning easterly Thurs AM. Very warm/dry with poor overnight RH recoveries expected through Sat with continued enhanced fire weather conditions. Additional critical fire weather conditions possible Mon/Tues with breezy westerly winds.

WA/ID/MT: Increasing fire weather threat as temperatures warm into the weekend with breezy westerly winds possible late weekend. Continued dry conditions with no precipitation expected (with exception of northern ID/MT Wed/Thurs).

AZ/UT/NM/southern CO: Monsoonal flow returning Thurs/Fri/Sat with scattered thunderstorms, rain becoming more numerous over the weekend, especially across AZ/UT with flooding possible.

Southern CA/southern NV: Very dry and warm through late week. Increasing showers/storms Sun/Mon with flooding possible.

Chip

A large, slow-progressing upper level trough has brought copious moisture to much of the northern Rockies and Eastern Great Basin. Even some winter weather advisories with very a cool air mass over MT. Widespread fire weather threats have been minimal across much of the western US and will likely remain quiet through the weekend. Despite the lack of optimal large fire conditions, several areas will need to be monitored for developing potential into mid-week.

As the trough begins to lift northeast into the northern Plains and Canada early week, another final shortwave will rotate around the apex on Monday. This will bring a slight chance of showers and thunderstorms across portions of NV/UT. With lack of low level moisture and drying taking place over the weekend, a few isolated dry thunderstorms are possible. No strong winds are anticipated outside of thunderstorm outflows. The threat will diminish Tuesday evening.

A big story will be developing, with upper level ridging along the West Coast and subsequent drying conditions and warming temperatures early week. These conditions will aggravate already drought stricken regions and provide ripe conditions for fire growth for CA and northward into OR/WA. No widespread thunderstorms or wind events are anticipated. However, temperatures above 90F and widespread RHs <20% with some <10% for central OR and northern CA by Wednesday will promote extreme fire behavior.

Weather Highlights:

NV/UT/southern ID: Isolated dry thunderstorms Mon/Tues. Becoming hot and mid-week.

OR/WA: A few scattered showers on Sun, lingering into Mon. Becoming very hot and dry Tues and continuing into late week.

CA: Warm and dry Sat/Sun. Becoming very hot and critically dry by mid-week.

Chip

Lack of widespread critical fire weather has kept fire starts down across much of the western US. Expect fire weather to become more critical today in OR/CA and spread eastward into ID/WY through late-week.

A cut-off upper level low off the coast of north CA will provide enough moisture and instability for scattered dry thunderstorms over high elevations. Very dry surface conditions combined with hot temperatures will make fuels very receptive to dry lightning. Tues, the low will drift southward along the coast, but lose ample moisture to provide widespread storms, however a few isolated storms are possible in central/north CA through Tues evening.

A more potent upper level trough will enter dig into the Pacific Northwest Tues, and begin to eject over the Northern Rockies on Wed. In advance of this trough, surface pressure falls are expected across central OR/WY creating breezy westerly winds Tues and Wed. Combined with warm temperatures and dry conditions, fire suppression will become difficult. Higher moisture across extreme northern WA/ID/MT will develop scattered rain and wet thunderstorms over higher elevations bringing some relief mid-week, with a higher heavy rain threat for eastern MT late week.

Surface low pressure will begin to develop across the central Plains in response to the eastward movement of these upper level features. With high pressure over the Rockies and the western US, a strong pressure gradient will set up over WY and southern ID, and northern CO late week. This gradient will develop breezy conditions across these areas leading to critical fire weather. In addition, increased upper level moisture will provide ample conditions for scattered dry thunderstorms across the same areas, especially over higher terrain. Cooler conditions will develop across the northern Rockies late weekend. Meanwhile, warm and dry conditions will continue for CA and western/central OR/WA with isolated locations of gusty winds through the weekend.

Weather Highlights:

CA: Critical fire weather Monday due to dry lightning, warm temperatures and dry conditions. Threat will lessen and drift southward on Tues/Wed. Continued hot and dry through the period.

OR/WA: Scattered dry thunderstorms across higher elevations of southern OR Monday. Increasing winds Tues/Wed with critical fire weather conditions expected Wed. A few wet thunderstorms over higher terrain of extreme nrthern WA.

Southern ID/WY/northern CO: Very breezy conditions developing with warm temperatures and dry conditions continuing into the weekend. Scattered dry thunderstorms possible through the period over higher terrain. Critical fire weather expected Wed through Fri.
MT: Isolated dry thunderstorms possible Wed through Fri for southern MT, with wet thunderstorm chances across northern MT. Otherwise warm and dry through Thurs with heavy rain possible Fri across eastern MT.

Chip