The cold-weather blues have hit and you long for sun and sand. But what are your chances of making a spur-of-the-moment Caribbean getaway? Even though it's the height of the winter season, they might not be as bleak as you think.

"We always have room for the impulse traveler," said Joan Medhurst, director of the Caribbean Tourism Association in New York that represents 26 countries. And this winter's fairly mild weather in the northeast -- at least so far -- may mean that fewer people are seeking out the warm islands.

But finding accommodations and flights during certain periods will not necessarily be easy. For example, the second and third weeks of February, especially the Washington's Birthday holiday, are generally heavily booked, and a recent spot check of island tourist boards and airlines indicates many hotels and some flights already have been heavily booked in March, particularly around Easter.

So you will have to be flexible. You may have to sign up for a vacation trip prepackaged by an airline or charter operator, instead of arranging your own itinerary (a percentage of seats and hotel rooms are allocated to these tours). The choice of available departure dates and accommodations will be limited. And if you can wait until March -- except around Easter -- you'll probably find more options both in available flights and places to stay. But it still is possible for you to wind up on your favorite island.

Even the cruise lines may have some vacant staterooms on Caribbean sailings in the next few weeks despite reports of very good business, according to Cruise Lines International, a trade association whose members' vessels embark from Miami, San Juan and other ports.

But how do you go about checking on hotel and airline space? You can contact the resorts and carriers directly, or work through your travel agent. In any case, you must be persistent.

Island representatives in this country emphasize that the booking situation changes daily and therefore they cannot keep current records. They suggest that travelers contact a travel agent, who can determine availability of hotels, flights and tour packages for specific dates. And they emphasize the need to keep trying to make reservations, because there are always late cancellations.

Most major airlines have tour desks, which also will book hotels or their own tour packages. Except for truly adventurous travelers, confirmed room reservations (preferably in writing) are a must this time of year. Obviously, unless you buy a package, last-minute departures leave little time for formal confirmations. That's another reason to reserve through a travel agent who can give you firm assurances -- and back you up if there's a problem after you arrive.

There's no sure-fire way to predict which islands are the best bets at the last minute. While well-known larger islands like Jamaica have the largest resort hotels, and generally more of all types of accommodations, they also draw the largest number of visitors. Smaller islands with more limited facilities should not be overlooked -- because fewer people know about them and because sometimes they're more difficult to get to and therefore draw fewer people. These include hideaways like St. Kitts and Nevis, St. Vincent and the Grenadines, St. Lucia, the Turks and Caicos -- and St. Eustatius (Statia) and St.-Barthe'lemy (St. Barts), the two newest members of the Caribbean Tourism Association.

A travel agent is probably the best source of booking information on smaller Caribbean islands, since many do not have representatives in this country; a number of the larger islands have information offices in New York City. The Caribbean Tourism Association (20 E. 46th St., New York, N.Y. 10017, 212-682-0435) provides general information and brochures. Some of the tourist offices offered comments about availability for the spur-of-the-moment traveler:

*Antigua: Major hotels were booked well in advance, but there is a little space. Last year set a record for arrivals, and this season will be even better.

*Bahamas: The traveler has several options with a number of airlines and charter operators to choose from. Airlines and tour operators have been allocated a major share of rooms for the prime months of February and March, so tourists are probably better off buying a package.

*Barbados: It's a good season with about 80 percent of the accommodations booked for February and March, the same as last year. There should be some space available both months.

*Bonaire: Travel from the U.S. was up 35 percent last year, but there should be some rooms because there is more space available.

*British Virgins: Business is very good, but there may be a smattering of rooms here and there.

*Cayman Islands: There is a slight chance of finding a room or a condo for a few days, but the hotel association says it has a waiting list dating back to October. It may even be impossible to find a seat on a flight.

*Curacao: It's fully booked in February, and March seems headed that way. So far it's an excellent winter season.

*Dominican Republic: Some rooms are still available around the island, particularly in the Puerto Plata area.

*French Caribbean (Martinique and Guadeloupe): February traffic will be quite heavy with March a little better. This winter there is a new booking service for small hotels -- nine are on Guadeloupe and 22 on Martinique with 20 to 30 rooms each. International Travel & Resorts (25 W. 39th St., New York, N.Y. 10018, 800-223-9815), which opened last fall, makes it possible for agents and individuals to confirm reservations at these 31 hotels without having to phone the islands.

*Grenada: Space is expected to be really tight until the last week in February, but then some rooms will become available through March. President Reagan is scheduled to make an official one-day visit on Feb. 20, to confer with Caribbean leaders and honor American servicemen killed during the U.S.-led invasion in October 1983. The island has made considerable progress since then. Its newest hotel, the Ramada Renaissance, opened Feb. 1 with about 130 rooms completed; 56 more rooms will be finished by the end of this winter.

*Haiti: You might find some rooms in February, and it could be a bit looser in March. Officials say that tourism on the island was hurt because Haitians were once identified as among the groups at high risk of contracting AIDS. The Center for Disease Control in Atlanta has rescinded that listing and the negative effect on travel has been reduced. Recently political unrest has caused an additional problem -- late last month the State Department advised American travelers not to go to Cap-Haitien, Gonaives and Les Cayes, scenes of anti-government protests. The three-day carnival season starts today in Port-au-Prince, the capital.

*Jamaica: There are rooms available. People are booking closer to departure dates, and anytime but Easter looks good; it makes sense to check for cancellations even during the holiday period. The island has 450 more rooms this year than during the same period last year. For spur-of-the-moment travelers, Air Jamaica has arranged a three-day, two-night "Jamaica Sampler" package with a tour operator in conjunction with its Monday afternoon flight from Baltimore to Montego Bay, through April 21. Rates depend on the hotel selected. The Sampler can be booked as late as the day of departure if seats are still available. It is sold only through travel agents.

*Puerto Rico: Officials expect a 5 percent increase in arrivals this year, marking a resurgence in tourism. Hoteliers are looking at an 80 to 95 percent occupancy rate this winter; but with 650 new rooms open, you can always find a room somewhere -- even if you have to leave San Juan for places like Ponce, Mayagu ez or San Germa'n.

*Trinidad and Tobago: They are almost fully booked for Carnival (Monday and Tuesday) and the George Washington and Easter holidays, but expect rooms to be available at other times.

*Turks and Caicos: Many people don't know about these small islands, and many of the 240 hotel rooms at Providenciales and Grand Turk have not been booked this February and March.

*U.S. Virgin Islands (St. Croix, St. John, St. Thomas): For anybody trying to make a quick getaway, it's going to be a little difficult. The hotel occupancy percentage in February should run in the high 90s. It may slow up a bit in March.

The following airlines are among those flying to the Caribbean. All say that seats are currently available to their destinations.

*American Airlines flies from New York's JFK Airport to the U.S. and British Virgins, St. Maarten, Antigua and Aruba. About 60 percent of American's passengers buy packages.

*Eastern, serving Puerto Rico, Barbados, the Dominican Republic, the French and Dutch Caribbean and other islands, with Miami as its major gateway, suggested that tourists use its vacation planning desk.

*TWA began its first service to Puerto Rico from New York last November and also flies from JFK to Nassau and Freeport in the Bahamas.

*Pan Am recently stopped service from New York to the French Caribbean but began serving the Dominican Republic and the Turks and Caicos from Miami. It also flies to a number of other islands. And a spokeman suggested that if you can't get to the island of your choice, Pan Am's daily nonstop to Mexico City from Dulles offers a gateway to Mexico's beaches, including the popular resort of Cancu'n on the Mexican Caribbean. (The Mexican Government Tourist Office said persistent travel agents should be able to find rooms there this season, though perhaps not at the major properties.)

Among tour operators offering packages departing from the Washington area, Wainwright's Travel Service indicated availability on some departures to Aruba, Jamaica, St. Thomas and Nassau. The Travel Committee's packages to St. Maarten, the U.S. Virgins, the Dominican Republic, Antigua and the Bahamas were at least 80 percent booked through February, but the firm suggests you check with your travel agent for the remainder of this month and March. Flyfaire's packages for the Bahamas and the Caribbean were reported still available for both February and March, with scheduled air via Eastern and Pan Am. These three operators book only through travel agents.

And if you decide you can wait just a little longer for your Caribbean winter break, those high-season hotel rates will drop drastically around mid-April.