Summary

October 17, 2019 - The discussion in the Looking Ahead section is a description of what the official national guidance from the National Weather Service (NWS) National Centers for Environmental Prediction is depicting for current areas of dryness and drought. The utilized NWS forecast products include the WPC 5-day QPF and 5-day Mean Temperature progs, the 6–10 Day Outlooks of Temperature and Precipitation Probability, and the 8–14 Day Outlooks of Temperature and Precipitation Probability – valid as of late Wednesday afternoon of the USDM release week. The NWS forecast Web page used for this section is http://www.cpc.ncep.noaa.gov/products/forecasts/.

A large upper-level low pressure system moved in the jet stream flow across the contiguous U.S. (CONUS) during this U.S. Drought Monitor (USDM) week, dragging surface lows and cold fronts along with it. Cooler air followed the fronts, bringing a colder-than-normal week to most of the country west of the Appalachians. Temperatures still averaged warmer than normal across the Southeast and parts of the Northeast. Above-normal precipitation accompanied the fronts and lows across the northern Plains, the central Plains to Mid-Mississippi Valley, and parts of Texas, the Great Lakes, and Southeast. Rain was moving along a stationary front across parts of the Southeast as the USDM week ended. Any rain that falls after the 12Z (7:00 a.m. EST) cutoff for this week’s USDM will be considered for next week’s map. Most of the West, parts of the central to southern Plains, and most of the Tennessee Valley to New England was drier than normal as the USDM week ended, with many of these areas receiving no precipitation. Soils continued to dry out in the Southwest, southern Plains, Ohio Valley, and East, and crops, pasture, and rangeland was in poor to very poor condition in more than 50% of the area in states in these regions. Streamflow was very low or near record low levels across the Southeast to southern New England. Precipitation deficits for the last 4 months of more than 10 inches below normal were common across the Southeast and parts of Texas, and 4-month deficits of 6 inches or more were evident across the Mid-Atlantic and Northeast regions. The dry conditions, coupled with increased evapotranspiration caused by unusually hot temperatures of the last couple months, resulted in very low values for drought indices such as the Standardized Precipitation Index (SPI) and Standardized Precipitation Evapotranspiration Index (SPEI). The streamflow, soil moisture, vegetation conditions, SPI, and SPEI were the basis for changes on this week’s USDM map.

Northeast

Abnormally dry (D0) conditions expanded in the Northeast with moderate drought (D1) pushing further into Pennsylvania and New Jersey and severe drought (D2) being introduced into Maryland and Delaware.

Southeast

Most of the region was drier than normal again this week. Abnormally dry (D0) to severe drought (D2) conditions expanded or intensified in parts of every state in the region, with extreme drought (D3) expanding in Georgia, South Carolina, Alabama, and Florida. Rainfall was above normal for the week in a few areas, with contraction of D0, D1, or D2 occurring in parts of Alabama and extreme western North Carolina. But for the most part, where rain did fall, it was only enough to temporarily halt further drought expansion.

South

Parts of Texas, Oklahoma, Arkansas, and Louisiana received over 2 inches of rain this week, which resulted in contraction of drought and abnormal dryness. Contraction also occurred in western Tennessee. But other areas continued dry, with expansion of drought and abnormal dryness occurring. Severe (D2) to extreme (D3) drought expanded in Texas and eastern Tennessee.

Midwest

It was wet along the Mid-Mississippi Valley and parts of the Great Lakes, but drier than normal across most of the rest of the Midwest region. Some contraction and some expansion of drought and abnormal dryness occurred along the Ohio Valley states, depending on where the rain fell and the change in moisture conditions. Most notably severe drought (D2) expanded in western Kentucky and adjacent southern Indiana, and extreme drought (D3) expanded in eastern Kentucky.

High Plains

In the High Plains region, the week was wetter than normal in the north and drier than normal in the south. There was no change in drought status in Colorado or Wyoming, but abnormal dryness and moderate drought expanded in Kansas with pockets of severe drought developing in southwest Kansas. Nebraska and the Dakotas continued free of drought and abnormal dryness.

West

Another week of no precipitation compounded dryness which has been developing over the last 6 months across Nevada and California, where abnormal dryness (D0) expanded. Intensifying dryness over the last 3 months prompted the expansion of D0 in the Pecos region of northeast New Mexico. Severe drought (D2) expanded in north central Arizona and adjacent south central Utah.

Alaska, Hawaii, and Puerto Rico

In Puerto Rico, 2+ inches of rain trimmed the moderate drought (D1) in the southwest and abnormal dryness (D0) in the east. But a strip of D0-D1 continued along the southwest to south central coast.

In Alaska, above-normal precipitation contracted moderate drought (D1) north of Anchorage and abnormal dryness (D0) along the Aleutian peninsula (Alaskan peninsula). In the panhandle, even though the last 60 days have been drier than normal, the southern half of Southeast Alaska (the panhandle) has been receiving fall rains and it has been helping to raise ground water levels. The area is still experiencing impacts (some hydro power generation with diesel power backup as a supplement) but the rains have helped to raise the levels of the lakes some. The SPI's for the last 3 months over that area is still below normal but not as low as it has been. As a result, the extreme drought (D3) in the southern panhandle was removed.

In Hawaii, some improvement occurred and some deterioration on most of the major islands. A heavy rain event last week mainly affected Kauai, Oahu, and the southeast-facing slopes of the Big Island, so improvement occurred along the leeward slopes. But on the Big Island, the southeasterly low level flow has worsened the drought along the Hamakua Coast. A rancher reported to the FSA that their pastures were in poor shape and they haven't had significant rainfall in several months. D0-D2 were expanded along the lower Hamakua Coast and windward Kohala Mountain slopes. Rain from afternoon deep convection contracted D1 near Kawaihae.

Pacific Islands

The weather pattern during this USDM week (10/09/19-10/15/19) was dominated by Super-typhoon Hagibis, which was moving westward away from the Northern Marianas early in the period. Hagibis subsequently recurved poleward and weakened somewhat before making its initial landfall on Japan’s Izu Peninsula (about 100 km southwest of Tokyo) as a Category-2-equivalent typhoon. About an hour later, Hagibis made its second landfall in the Greater Tokyo Area. Despite having weakened from a Category-5 to a Category-2 typhoon, there were still dozens of fatalities (as of this writing), torrential rains, flooding, mudslides, and high winds.

The monsoon trough remained active over the U.S. API region, with embedded weak cyclonic circulations moving through it. Early in the drought monitor week, one of these rotary circulations was centered near Pohnpei, with a surface trough extending east-northeastward to beyond the Date Line. Showers and thundershowers were focused along and just north of this surface front in an area of low-level convergence featuring flow from the east and northeast. The same general flow pattern continued through the end of the drought week, although it weakened substantially. South of the equator, several surface troughs helped maintain a wet and unsettled weather pattern in the vicinity of American Samoa.

Satellite-based estimates of 7-day precipitation (QPE) showed two areas of heaviest rainfall. The first was along and near the track of Super-typhoon Hagibis (3-8 inches, locally greater) over the eastern Philippine Sea. The second area of heavy rainfall (2-6 inches, locally greater) was confined to a band that stretched east-northeastward from well east of Palau across portions of Micronesia (FSM) to the northern Marshall Islands (RMI) and beyond. This southern band of heavy rainfall was due to persistent surface troughs and embedded weak cyclonic circulations, low-level convergence, and locally exacerbated by mountainous terrain.

The International Airport in Palau reported 1.11 inches of rain during the past drought week (Oct 9-15), which falls well short of the 2-inch weekly minimum for fulfilling most water requirements in this area. According to the NWS Forecast Office in Palau, Koror’s (preliminary) year-to-date rainfall total stands at 95.43 inches, and the normal for this period is 117.52 inches. This places Koror’s rainfall total to date at 81.2 percent of normal.

For Guam and the Commonwealth of the Northern Mariana Islands (CNMI), weekly precipitation totals (ending October 15th) include 1.76 inches at Guam, and anywhere from 0.24-inch to 1.10 inches at Saipan (0.24-inch at a National Park Service site, 0.44-inch (ASOS), and 1.10 inches at a manned gauge). Rota reported 1.19 inches of precipitation during the past week, which exceeds the 1-inch weekly minimum rainfall amount needed to meet most water needs. For nearly every week since July 30th, Guam and the CNMI have received sufficient rainfall amounts, where these amounts exceeded the given 1-inch per week threshold. Additionally, these same areas received sufficient rainfall for the months of July, August, and September, where rainfall totals for each month exceeded the 4-inch minimum threshold. There was no reason to change the DM drought depiction this week.

In the Federated States of Micronesia (FSM) Yap reported 3.85 inches of rain this week (2 days missing), which is nearly double the minimum weekly amount of 2 inches needed to meet most water needs. Its depiction remains unchanged. Woleai received 0.75-inch of rain this week (2 days missing), far short of the 2-inch threshold. Its depiction remains unchanged this week from D0(S). Chuuk Lagoon received 2.90 inches of rain this week, comfortably surpassing the 2-inch threshold across the FSM. Since June, each month’s rainfall has exceeded the 8 inches per month threshold for meeting water needs. No change was made to its drought-free designation. The stations of Lukunor, Nukuoro, and Kapingamarangi received 3.41 inches (1 day missing), 3.77 inches (1 day missing), and 1.50 inches of precipitation (1 day missing), respectively, with the first two stations easily surpassing the 2-inch minimum rainfall requirement for the week. Fifty percent or more of the months dating back to November 2018 have seen more than adequate rainfall (>8 inches per month). Therefore, the D0(S) designation is retained at these three stations. Across eastern Micronesia, the stations of Pohnpei, Pingelap, and Kosrae reported weekly rainfall amounts of 12.14 inches, 2.80 inches, and 1.69 inches. This shows how localized and intense convective tropical rainfall can be. This is likely related to the complex interaction between surface troughs, weak circulation centers embedded within the monsoon trough, and mountainous terrain. Incidentally, Pohnpei reported greater than 8 inches of rain for nearly every month dating back to November 2018, with the one exception of February 2019 when 7.57 inches of rain fell (which can hardly qualify as a “dry” month). The drought designations at these three stations remain unaltered this week.

In the Republic of the Marshall Islands (RMI), weekly rainfall amounts ranged from no rain in Ailinglapalap (2 days missing) to 5.12 inches in Kwajalein. Intermediate amounts include Mili (0.09-inch, 2 days missing), Utirik (0.35-inch, 2 days missing), Jaluit (0.46-inch, 2 days missing), Majuro (1.17 inches, 1 day missing), and Wotje (2.18 inches, 2 days missing). None of the drought designations within the RMI were changed this week, even for those stations which received little rainfall this week, thanks to longer-term wetness. Exceeding the 2-inch weekly minimum rainfall amount at Wotje helps to offset additional deterioration, resulting in the retention of its D0(S) status. From November 2018 to September 2019, there was a 6 month consecutive period where Wotje did not receive the minimum 8-inch monthly rainfall requirement for meeting most water needs.

American Samoa continues to display interesting differences in rainfall distribution. Expert on-site feedback (which includes photographs) suggests conditions on the island are more lush than previously thought. This resulted in the recalibration of minimum rainfall criteria necessary to address water needs in American Samoa. We have revised the minimum amount of rainfall needed (to maintain adequate water supplies) to 1-inch per week, and 4-inches per month, which is half the value of the criteria used previously. Pago Pago reported 1.42 inches of rain this week (3 days missing), and Toa Ridge observed 1.32 inches of rain (5 days missing), both of which now qualify as “wet” weeks (having exceeded the new 1-inch/week threshold). In contrast, Siufaga Ridge reported only 0.20-inch of rain this week (2 days missing). The reassessment of conditions on American Samoa this week resulted in changing the drought designation at Tutuila from D0(S) last week to D-nothing this week.

Virgin Islands

According to the NWS Forecast Office in San Juan, Puerto Rico, the Cyril E. King Airport in St. Thomas reported 0.40-inch this week and 0.44-inch of rain since October 1st. Observed rainfall totals of 3.95 inches and 26.88 inches were reported at the King Airport since September 1st and January 1st, respectively. These values are 49.7 and 95.7 percent of normal, respectively. Data from the CoCoRaHS station at Anna's Retreat (VI-ST-1) indicated 0.23-inch of rain during the past week, and a total of 2.67 inches of rain since the start of September. The Grade School-3 Well level dropped about a foot during the past week, and is now about 8 feet below the land surface. For Charlotte Amalie, SPI values at 1(-0.51), 3(+0.80), 6(+0.01), 9(-0.04), and 12(-0.26) months support drought-free conditions.

The Henry E. Rohlsen Airport in St. Croix reported a trace of rain during the past week, and 0.02-inch so far in October. Observed rainfall amounts since September 1st and January 1st are 1.90 inches (27.0 percent of normal) and 18.38 inches (67.6 percent of normal), respectively. The CoCoRaHS station at Christiansted (VI-SC-8) indicated 0.08-inch of rain this past week, and a total of 6.36 inches since September 1st. Groundwater levels at the Adventure 28 well continued to be low this week, following a general decline that began in late 2018. For Christiansted, SPI values at 1(-1.07), 3(-0.34), 6(-0.16), 9(-0.85), and 12(-1.46) months, below normal rainfall, and low well levels all favor a 1-class downgrade in DM category, from D-nothing to D0(S). This comes at a time when we are at the heart of the wet season across the Virgin Islands.

According to data provided by a volunteer observer, Windswept Beach on St. John's received 0.05-inch of rain this past week, with a year-to-date total accumulation of 28.52 inches. Like St. Thomas, the short-term well trend shows falling well levels on St. John, with current water levels similar to, or a bit lower than, those observed immediately after the late-August tropical downpours. SPI values at 1(-0.9), 3(+0.3), 6(+0.11), 9(-0.35), and 12(-0.92) months suggests deterioration may be needed soon for St. John.

Looking Ahead

Since the Tuesday morning cutoff for this week’s USDM, several inches of rain have fallen along the frontal zone in the Southeast, a low pressure and frontal system was bringing rain to the Northeast, and another system was bringing precipitation to the Pacific Northwest. This precipitation will be incorporated into next week’s USDM. For October 17-22, Pacific frontal systems will bring 3 or more inches of rain to the western mountain ranges of Oregon and Washington with an inch or more to the northern Rockies and half an inch from the Pacific Northwest to Montana and Wyoming. A large area of an inch or more of precipitation is forecast to fall along the Mississippi River to eastern portions of Kansas, Oklahoma, and Texas, and into the western Great Lakes and northern Plains. Half an inch to an inch and a half of rain may fall in the Ohio and Tennessee Valleys, 2 or more inches in much of the Northeast, and 1-2 inches across the Southeast, with up to half an inch across the rest of the Great Lakes. But the Southwest into the southern and central High Plains are forecast to get no precipitation. Temperatures are expected to moderate, forecast to be near to above normal across most of the CONUS. For October 22-30, odds favor above-normal precipitation across the East Coast into the Great Lakes, and for part of the period along the northern Rockies to northern Great Plains. The period will likely be drier than normal across the Southwest into the southern and central Plains, eventually extending to the Mississippi Valley later in the period. Odds favor drier-than-normal weather in western Alaska with wetter-than-normal weather in the south and east. The temperature outlook for October 22-30 is warmer-than-normal along the West Coast and East Coast, with colder-than-normal weather from the Rockies to Appalachians. The period is expected to be warmer than normal along western and southern Alaska.

Author(s):

Richard Heim, NOAA/NCEI
Anthony Artusa, NOAA/NWS/NCEP/CPC