####018007302#### FXUS66 KEKA 031251 AFDEKA AREA FORECAST DISCUSSION National Weather Service Eureka CA 551 AM PDT Fri May 3 2024 .SYNOPSIS...A cold front will move across the area this evening through early Saturday morning bringing a period of heavy rain. Brief heavy wet snow is also expected above 2500 feet early Saturday morning. Colder air will settle over the area for the remainder of the weekend with frost and freezing temperatures expected for the interior valleys during early morning hours. && .DISCUSSION...Prefrontal light rain is expected to begin over Del Norte County late this afternoon. Heavy rain will develop this evening for Del Norte and northern Humboldt as a surface cold front moves southward by midnight or shortly after. Heavy rain rates >0.30in/hr are probable (>70% chance). Chance for hourly rates >0.50in/hr are 10-20% for the interior mountains Del Norte and Humboldt, except for the King Range where it is 40%. These heavy rates (>0.50in/hr) per the HREF are not expected to sustain over a large area (an entire basin) for more than hour or two and thus will hold off on hoisting a flood watch. Minor nuisance flooding of low-lying areas with poor drainage and ponding water on roads are certainly possible with these heavy rates. It is quite anomalous for early May and will continue to highlight the heavy rain in a weather story graphic and social media post. Flows on the rivers are low and April was not exactly wet with above normal precip, so soils are not saturated as earlier this winter. There will be run-off concerns and advisories may be necessary once the heavy rain commences by late this evening. The cold front will move SE by early Saturday and southern Mendocino and southern Lake Counties will also get a good soaking of rain, 0.50-0.75in over 6-12 hours, mostly Saturday morning. Threat of widespread heavy rain will diminish rapidly by midday Saturday after the brief influx of moisture shifts southward and an unseasonably cold airmass settles over the area. Looking at the HREF, snow levels are forecast to rapidly fall early Saturday with probabilities >70% for hourly snow rates > 1in/hr. This warrants at least a winter weather advisory for elevations above 2500 feet. Greatest uncertainty is with snow levels. Levels could vary to as low 2000 feet to as high as 3500 feet. Leaned toward lower snow levels, 2500 ft, due heavy precip rates driving the freezing levels lower. Impacts over the major highway passes will be offset the high May sun angle and relatively warm road surface. Reduced visibilities from heavy snow and slick roads will have impacts and a winter weather advisory has been hoisted til 11 AM Saturday. Shower activity will continue during the day on Saturday as the cold core aloft moves overhead and onshore westerlies bring open cellular shallow convection onto the coast. Not all that confident in the potential for low topped thunderstorms with surface temperatures cooling down drastically in the interior and 850mb temps dropping to -3C. High temps for the warmest valleys will struggle to reach 60F. After the precip winds down or diminishing subfreezing temperatures will be primary concern for the interior Sunday morning. It is certain (100%) for the mountains. Valleys will most likely freeze too (>70% chance), mostly in Trinity County. Spreads are much wider with probabilities from 30% to 70% for valleys in Del Norte, Humboldt, Mendocino and Lake. Frost or mid 30's are much more certain, even for coastal areas. Fog and low clouds will probably form and hinder the longwave cooling in some valleys, especially after 1 to 3 inches of rain. Another shortwave trough in NW flow will probably generate more showers Monday (highest chances Del Norte and northern Humboldt). This trough will not be as cold and moisture will be limited. Surface northerlies will begin to strengthen after passage of this shortwave trough and dry conditions are most likely into mid week (>80% chance). Majority of the 500mb ensemble cluster means (>80%) leads to more confidence in dry weather with slow warming of daytime temperatures. The one caveat to the "warming" will be potential for freezing early morning temps in the interior valleys under clearer skies and calm or light winds, assuming absolutely no fog and low clouds. Also, the strength of E-NE winds will factor into "how warm" and the eventual return of coastal stratus. High temps in the low 90's for warmest valley are not completely out of the realm of possibilities either, increasing to 20-40% on Friday. Interior valley high temps in the 80's are much more probable (>60%) by Friday. DB && .AVIATION...VFR prevailed at all terminals this morning. Nighttime fog product showed extensive low clouds and fog in the interior valleys. The LIFR conditions (1/4SM VV002 at FOT for example) will most likely not have long duration impacts at any of the terminals this morning. It may spill out from the valleys around daybreak and get close to KACV resulting in brief LIFR. Otherwise, VFR is forecast to prevail today as multi-layered clouds with an approaching front increase through the day. Conditions are forecast to deteriorate to IFR in moderate to heavy rain this evening at KCEC and KACV. IFR conditions will take longer to envelope UKI where LLWS should return with frontal passage by 12Z Sat. DB && .MARINE...Northerly winds have diminished in response to an approaching front. Southerlies are expected to develop this morning and slowly increase through the day today ahead of the cold front. Southerly gusts up to 30 kts are forecast with the passage of the front, particularly in the northern inners this evening, then sharply shift to west behind it's passage. Gusts up to 25 kts are expected with passing convective showers into Saturday morning, slowly becoming restricted to the southern outers by Saturday afternoon. Winds weaken Saturday night and turn westerly Sunday. Another weak frontal passage may pass early next week, then more robust northerlies are possible by midweek. Seas remain low and locally generated through this weekend, and a series of NW swells are expected to fill in early next week. DB/TDJ && .EKA WATCHES/WARNINGS/ADVISORIES... CA... Winter Weather Advisory from 4 AM to 11 AM PDT Saturday for CAZ102-105>108-111. Frost Advisory until 8 AM PDT this morning for CAZ107-108. NORTHWEST CALIFORNIA COASTAL WATERS... Small Craft Advisory from 9 PM this evening to 3 AM PDT Saturday for PZZ450. Small Craft Advisory from 9 PM this evening to 9 AM PDT Saturday for PZZ470. Small Craft Advisory from 3 AM to 9 PM PDT Saturday for PZZ475. && $$ Visit us at https://www.weather.gov/eureka Follow us on Facebook and Twitter at: https://www.facebook.com/NWSEureka https://www.twitter.com/NWSEureka For forecast zone information see the forecast zone map online: https://www.weather.gov/images/eka/zonemap.png ####018008288#### FXUS61 KBOX 031254 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 854 AM EDT Fri May 3 2024 .SYNOPSIS... High pressure supplies dry weather with cooler onshore breezes today and Saturday. Increasing clouds Saturday night with showers likely and cooler temperatures Sunday into Sunday night. High pressure then brings dry and warm weather Monday afternoon into Tuesday. Unsettled weather is possible later Wednesday into Thursday, with the risk of showers but likely remaining mild. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... A blanket of low clouds across the region, with the exception being over western MA/CT, where breaks in the overcast are occurring per latest visible satellite imagery and surface observations. This low level moisture is trapped beneath subsidence inversion across the area, courtesy of 1020+ maritime high. However, strong May sunshine will slowly erode the clouds and give way to breaks of afternoon sunshine, especially mid to late afternoon. Seasonable temps but feeling cooler across eastern MA with maritime NE flow, capping highs in the 50s this afternoon, response to water temps only in the 40s. Also, low level NE jet currently yielding gusts up to 25 mph over Cape Cod and the Islands, will diminish this afternoon. Hence, less wind this afternoon than this morning. Previous forecast captures these details nicely, therefore no major changes with this update. Earlier discussion below. Previous Discussion... A broad anticyclone centered to the north/northeast of southern New England takes control today. A steady northeast flow will advect a cooler air mass over southern New England this afternoon with 925 hPa temps dropping to 5C or lower. With diurnal mixing this will translate to surface temps in the low to mid 60s across the interior and mid to upper 50s along the coast. Downsloping from easterly flow over The Worcester Hills will support warmer temperatures in the upper 60s in the CT River Valley. After a cloudy start, a mid-level ridge axis building in from the west will be accompanied by a much drier air mass that will allow some clearing my late morning/early afternoon. Still expect FEW to SCT diurnal clouds this afternoon, but there should be a good amount of sunshine as well. Overall a seasonable day in southern New England. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... Tonight and Tomorrow High pressure supports dry/quiet weather across southern New England tonight. Winds become light and variable which should allow for fairly efficient radiational cooling across the region, though there is a question as to how much cloud cover there will be and thus cooling potential. Decided to blend the NBM25th percentile with CONSMOS to yield low temps in the low to mid 40s which is near to slightly below normal for early May in southern New England. High pressure remains in control on Saturday, so we continue to expect dry/quiet weather. However, continued onshore flow will support cloudiness across the region. Very little change in the air mass for Saturday, so expect high temperatures similar to what should be observed Friday afternoon with cooler temps along the coast in the mid 50s and warmer temps across the interior and CT River Valley ranging from the low to upper 60s. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Highlights: * Cool but dry Saturday night into Sunday, then numerous showers Sunday afternoon into Monday morning. * Dry & warm Monday afternoon into Tuesday * Showers possible late Wed/Wed night into Thursday. Synoptic Overview... Tranquil stretch of weather in response to east coast ridging/above normal heights much of this forecast period. A brief interruption to the pattern, as northern stream trough briefly erodes this ridge with a risk of showers later Sunday into Monday morning. Then dry post frontal NW flow returns later Monday and into Tue. Progressive pattern follows with ridge building back into the northeast, but may be accompanied by warm frontal showers later Wed/Thu. Temperatures... Cool maritime ridge Sat night into Sunday, with lows in the 40s and highs 55-60. 850 mb ensemble temp anomalies peak 18z Monday over southeast MA, in the transition to post frontal. Thus, looking at a warm day (provided clouds & showers exit before the PM hours) with WNW flow combined with warm temps aloft coupled with downsloping winds. Hence, coastline warms up too, including Cape Cod and Islands. Also, dew pts in the 50s will provide mild/warm feel to the airmass. Not much of a drop off in temps in the post frontal airmass, thus, another warm day Tuesday with NW flow, warm temps aloft combined with downsloping NW winds. Likely not as warm Wed/Thu but probably above normal(60s). Precipitation... High amplitude, deep layer ridge with 1030 mb high from the maritimes into eastern MA, provides dry weather Sat night into Sunday morning. Then approaching trough provides cyclonic flow and increasing/above normal PWATs yield numerous showers Sunday afternoon into Monday morning. Ensembles offering likely probs (60- 70% for 0.25 inches of rainfall, with chance probs (30-40%) for up to 0.50 inches of rainfall. Timing for widespread showers, highest PWATs from 06z-12z Monday per ensembles. Post frontal with dry slot during Monday afternoon. Drier than normal PWATs linger into Tue, then becoming unsettled Wed/Thu with moisture plume advecting across the region (PWATs up to 200% of normal). Too far out in time for details, but looking at height anomalies from the ensembles, anomalous lows over Newfoundland and the Dakotas, may result in a slower ridge advecting across SNE mid of next week. Thus, high amplitude pattern may support dry weather lingering thru much of Wed, especially eastern MA/RI. && .AVIATION /13Z FRIDAY THROUGH TUESDAY/... Forecast Confidence Levels: Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 12Z TAF Update: Through 12Z...High Confidence in trends, but lower confidence on exact timing of improving conditions. IFR/MVFR cigs this morning slowly improves to VFR this afternoon and persist into the evening push. Gusty NE winds up to 20 kt across Cape Cod and Islands at 11z, diminishes this afternoon. Earlier discussion below. ------------------------------------------------------------------- Tonight...High confidence VFR. Light and variable winds. Tomorrow...High confidence VFR. East winds from 5 to 10 knots. KBOS TAF...High confidence KBDL TAF...High confidence Outlook /Saturday Night through Tuesday/... Saturday Night: VFR. Chance SHRA. Sunday: Chance SHRA. Sunday Night: Breezy. Chance SHRA. Monday: Chance SHRA. Monday Night through Tuesday: Slight chance SHRA. && .MARINE... Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. Marine Today through Tomorrow High pressure supports fairly calm conditions across the coastal waters through Saturday. Winds will prevail out of the northeast from 10 to 15 knots with some gusts to 20 knots possible. Seas generally 1 to 3 feet. Outlook /Saturday Night through Tuesday/... Saturday Night through Sunday: Winds less than 25 kt. Chance of rain showers. Sunday Night through Monday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers. Monday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers. Tuesday: Winds less than 25 kt. Slight chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Nocera/RM NEAR TERM...Nocera/BW/RM SHORT TERM...RM LONG TERM...Nocera AVIATION...Nocera/RM MARINE...Nocera/RM